Radio access networks in which mobile devices can be scheduled to use the same time-frequency resource

ABSTRACT

An example communication system in a cellular network comprises: a processing system comprising a controller and remote units, with the remote units being configured to communicate with the controller and to communicate with mobile devices within a communication cell of the cellular network. At least part of the processing system is configured to perform operations comprising: estimating signal strength experienced by all or some of the mobile devices; identifying, based at least on the signal strength, one or more of the mobile devices that can be scheduled for communication with one or more of the remote units in the communication cell on a same airlink resource; and scheduling the communication.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. application Ser. No.16/040,253, filed Jul. 19, 2018, and titled “RADIO ACCESS NETWORKS INWHICH REMOTE UNITS ARE CONFIGURED TO PERFORM AT LEAST SOME BASEBANDPROCESSING,” issued on Jan. 14, 2020 as U.S. Pat. No. 10,536,959, whichis a continuation of U.S. Pat. No. 10,057,916, issued Aug. 21, 2018, andtitled “RADIO ACCESS NETWORKS IN WHICH MOBILE DEVICES IN THE SAMECOMMUNICATION CELL CAN BE SCHEDULED TO USE THE SAME AIRLINK RESOURCE,”which claims priority to U.S. Provisional Application No. 62/009,653,filed on Jun. 9, 2014 and to U.S. Provisional Application No.62/051,212, filed on Sep. 16, 2014. The contents of U.S. applicationSer. No. 16/040,253, U.S. application Ser. No. 14/734,311, U.S.Provisional Application Ser. No. 62/009,653, and U.S. ProvisionalApplication Ser. No. 62/051,212 are all incorporated herein byreference.

TECHNICAL FIELD

This disclosure relates to radio access networks (RANs).

BACKGROUND

The widespread use of mobile devices, such as smartphones, has increasedthe demand for mobile data transmission capacity and for consistent andhigh-quality radio frequency (RF) coverage at in-building and otherdensely populated locations. Traditionally, inside buildings, mobileoperators rely on a Distributed Antenna System (DAS) to allow users toconnect to the operators' networks for voice and data transmission.

SUMMARY

An example communication system in a cellular network comprises: aprocessing system comprising a controller and remote units, with theremote units being configured to communicate with the controller and tocommunicate with mobile devices within a communication cell of thecellular network. At least part of the processing system is configuredto perform operations comprising: estimating signal strength experiencedby all or some of the mobile devices; identifying, based at least on thesignal strength, one or more of the mobile devices that can be scheduledfor communication with one or more of the remote units in thecommunication cell on a same airlink resource; and scheduling thecommunication between the one or more mobile devices and the one or moreremote units. The example communication system may include one or moreof the following features, either alone or in combination.

The airlink resource may include a frequency band. At least somecommunication among at least some of the controller, the remote units,and the mobile devices may occur using radio frequency (RF) signals,where at least some of the RF signals represent information destinedfor, or originating from, a mobile device. The remote units may beconfigured to communicate with the mobile devices using RF signals. Thecontroller may be configured to estimate the signal strength experiencedby a mobile device, and to represent the estimate numerically, with theestimate corresponding to transmission or reception needs of the mobiledevice for which the estimate was made. The controller may comprise areal-time scheduler or another type of scheduler configured to performscheduling for the mobile device based on the estimate.

The transmission or reception needs of the mobile device may correspondto estimated signal loss between the mobile device and one or more ofthe remote units. The estimated signal loss may be based on an uplinktransmission from the mobile device to one or more of the remote units.The uplink transmission may be based on an LTE Sounding Reference Signal(SRS) transmission. The uplink transmission may be based on an LTE PRACHtransmission. The uplink transmission may be based on an LTE PUCCHtransmission. The uplink transmission may be based on an LTE PUSCHtransmission. The transmission or reception needs of the mobile devicemay be based on traffic load experienced by one or more of the remoteunits. The estimate may be represented using numerical values that arebased on one or more measurements on an uplink from a mobile device to aremote unit. The estimate may be represented using numerical values thatare 0 or 1. The estimate may be represented using numerical values thatare values selected from a finite number of levels greater than two.

An estimate of signal strength for a mobile device may be representedusing numerical values. For the mobile device, the numerical values mayform a quantized signature vector. The controller may be configured toperform operations comprising: determining, based on quantized signaturevectors for the mobile device and at least one other mobile device thatthe mobile device and the at least one other mobile device can bescheduled on the same airlink resource for communication. The quantizedsignature vector may be based on a threshold signal-to-interference-plusnoise ratio (SINR). Two mobile devices can be scheduled on a samefrequency band in response to a sum of quantized signature vectors forthe two mobile devices having no component that exceeds a presetthreshold. The numerical values may be based, at least in part, on alocation of the mobile device within the communication cell.

At least two of the remote units may be configured so that, when two ormore mobile devices are scheduled for communication on a same airlinkresource, different ones of the remote units communicate with differentmobile devices on the same airlink resource. At least one of the remoteunit may be configured so that, when two or more mobile devices arescheduled for communication on a same airlink resource, the at least oneof the remote units does not communicate with any mobile device.Alternatively, the at least one of the remote units may be capable ofcommunicating with multiple mobile devices at simultaneously. The atleast one remote unit may be configured to communicate using reducedtransmit power.

The controller may be configured to cause communication with a mobiledevice to occur at a transmission power that is below a standardtransmission power in a case where the mobile device is within aspecified distance of a remote unit. The controller may be configured todetermine if the mobile device is within the specified distance based onone or more measurements of uplink transmissions of the mobile device atone or more remote units. The uplink transmissions may comprise one ormore of: LTE SRS, PUCCH, PRACH or PUSCH transmissions.

The operations performed by the processing system may comprisedetermining a bit rate at which data is to be transmitted to and from amobile device. The operations performed by the processing systemcomprise determining a bit rate for communication between a mobiledevice and a remote unit. Operations for determining the bit rate maycomprise: receiving, from the remote unit, information about one or moremeasurements on an uplink control channel between the mobile device andthe remote units; and using the one or more measurements to determinethe bit rate. The bit rate may be based on uncertainty due tosmall-scale fading.

The operations performed by the processing system may comprisedetermining a bit rate for communication between a mobile device and aremote unit. Operations for determining the bit rate may comprise:receiving, from the mobile device, feedback information about success orfailure of past data transmissions; and using the feedback informationto determine the bit rate. The feedback information may comprise HybridARQ (HARQ) feedback. In a case that a dominant interferer of the mobiledevice has changed, past HARQ feedback need not be used when determiningthe bit rate.

The operations performed by the processing system may comprisedetermining a bit rate for communication between a mobile device and aremote unit. Operations for determining the bit rate may comprise:receiving, from mobile devices, channel state information (CSI)feedback; and using the CSI feedback to determine the bit rate.

The operations performed by the processing system may comprisedetermining a bit rate for communication between a mobile device and aremote unit. Operations for determining the bit rate may comprise:receiving, from mobile devices, channel state feedback including aninterference measurement; and using channel state feedback including theinterference measurement to determine the bit rate. The interferencemeasurement may be based on an LTE Channel State Information ReferenceSignal (CSI-RS). The interference measurement may be reported by amobile device that is configured to report interference measurements fordifferent interference scenarios.

In the example communication system, a same airlink resource may be usedfor downlink transmission from one or more remote units to a mobiledevice. In the example communication system, a same airlink resource maybe used for uplink transmission from a mobile device to one or moreremote units. In the example communication system, a same airlinkresource may be used uplink transmission from a mobile device and one ormore other mobile devices to one or more remote units for which signalsare jointly processed.

In the example communication system, an estimate of signal strength isrepresented using numerical values. For a mobile device, the numericalvalues may form a quantized signature vector. The controller may beconfigured to perform operations comprising: determining that thequantized signature vector is orthogonal to another quantized signaturevector by performing a logical operation using the quantized signaturevector and the other quantized signature vector

The operations performed by the processing system may comprisedetermining which remote units are to communicate which mobile devicesbased, at least in part, on locations of mobile devices within thecommunication cell. The operations performed by the processing systemmay comprise dividing the communication cell into virtual cells suchthat different mobile devices in at least two different virtual cellsare configured for communication on the same frequency; and for a mobiledevice at a border of first and second virtual cells, controlling afirst remote unit in the first virtual cell to transmit to the mobiledevice at a non-maximum power level and controlling a second remote unitin the second virtual cell to transmit to the mobile device at anon-maximum power level.

The operations performed by the processing system may comprisedetermining bit rates at which communications are to be transmittedbetween the two or more mobile devices and the two or more remote units.Operations for determining a bit rate for communication between a mobiledevice and a remote unit may comprise: receiving, from all (or a subsetof) remote units in the communication cell, information on an uplinkcontrol channel, where the information corresponds to a predicted signalstrength for the mobile device in the communication cell, where thepredicted strength deviates from an actual signal strength for themobile device in the communication cell, and where the predicted signalstrength is associated with a first bit rate; and reducing the first bitrate based on the actual signal strength to produce a second bit ratefor communication between a mobile device and a remote unit.

The remote units may be configured to perform at least some basebandprocessing. The at least some baseband processing may include receivingand extracting the information on an uplink control channel. Theoperations performed by the processing system may compriseload-balancing communications to remote units on the uplink controlchannel. Operations to perform the load-balancing may comprise settingperiods and phases for transmissions from some mobile devices so as notto overlap with transmissions of other mobile devices. Operations toperform the load-balancing may comprise setting periods fortransmissions from some mobile devices based on a communication trafficload in the communication cell.

In the example communication system, communication may be on a downlinkfrom the two or more remote units to two or more mobile devices. Thecommunication may be on an uplink from the two or more remote units totwo or more mobile devices.

The mobile devices may comprise a first mobile device and a secondmobile device, and identifying one or more of the mobile devices thatcan be scheduled for communication may comprise identifying that thefirst and second mobile devices can be scheduled for communication on asame frequency on an uplink. The uplink may comprise at least one of LTEPUCCH or PUSCH channels.

The operations performed by the processing system may comprise dividingthe communication cell into virtual cells such that different mobiledevices in at least two different virtual cells are configured tocommunicate on a same frequency. The same frequency may comprise a partof a larger frequency band. The different mobile devices may beconfigured also to communicate over different frequencies within thelarger frequency band.

The controller may comprise a first controller and the processing systemmay comprise one or more second controllers. The first controller maycoordinate operation of the one or more second controllers. Each of theremote units may be configured to communicate with a correspondingsecond controller and to communicate wirelessly with mobile devices. Thefirst controller may implement a central coordination function tocontrol operations of the one or more second controllers.

The operations performed by the processing system may comprisedetermining locations of the mobile devices within the communicationcell; and scheduling communication between two or more mobile devicesand two or more remote units so as to selectively allocate resources inthe two or more remote units.

The operations performed by the processing system may comprise loadmanagement of an uplink control channel processing load on the remoteunits. Operations to perform load management may comprise adjustingperiods for transmissions from some mobile devices based on acommunication traffic load in the communication cell. The communicationtraffic load may be based on the number of connected users.

An example communication system comprises: remote units to communicatewith mobile devices using radio frequency (RF) signals, where the RFsignals include information destined for, or originating from, a mobiledevice; and a controller comprising a real-time scheduler configured toassign airlink resources to mobile devices for communication. The remoteunits may be configured to perform at least some baseband processing,with the at least some baseband processing including receiving andextracting the information on an uplink control channel. The at leastsome baseband processing may be spread among multiple remote units,where the at least some baseband processing includes operationscomprising setting periods and phases for transmissions from one or moremobile devices so that transmissions from the one or more mobile devicesdo not overlap with transmissions of one or more other mobile devices.The example communication system may include one or more of thefollowing features, either alone or in combination.

The setting of periods and phases may be for uplink control channeltransmissions of the one or more mobile devices and may be based on oneor more remote units that process the uplink control channeltransmissions. The setting of periods and phases may be for uplinkcontrol channel transmissions of the one or more mobile devices and maybe based on a change in a remote unit processing of the uplink controlchannel transmissions. The uplink control channel transmissions maycomprise Scheduling Request (SR) or Channel State Information (CSI)transmissions.

An example communication system comprises remote units to communicatewith mobile devices using radio frequency (RF) signals, with at leastsome of the RF signals including information destined for, ororiginating from, a mobile device; and one or more processing devices toexecute instructions to implement components comprising: two or morecontrollers, with the two or more controllers comprising real-timeschedulers to assign airlink resources to one or more mobile devices forcommunication with one or more of the remote units; and a coordinator tocoordinate assignments made by the real-time schedulers. The examplecommunication system may include one or more of the following features,either alone or in combination.

The coordinator may be part of one of the controllers. Each mobiledevice may be managed by one of the controllers. At least one of theremote units may be configured to demodulate and to decode PRACHtransmissions. One or more of the controllers may be configured tomanage a mobile device determined by the at least one remote unit. Oneor more of the controllers may be configured to operate as a backhaulcontroller to manage connection to an external network and, uponreceiving a page for a mobile device, to manage a mobile device. One ormore of the controllers may be configured to operate as a timing sourcefor one or more of the remote units.

Two or more remote units may be part of a cell. One or more of thecontrollers may be configured to serve mobile devices in the cell viaone or more of the remote units. One or more of the controllers may beconfigured to provide data for downlink common channels for one or moreof the remote units.

At least some mobile devices may be configured to receive data on two ormore frequency carriers. Each controller may be configured to manage oneof the frequency carriers, and each controller may be configured toserve a mobile user corresponding to a carrier of the controller. Thecoordinator may be configured to coordinate airlink resource assignmentsacross multiple frequency carriers. A remote unit may be configured toreceive data from more than one controller. The remote unit may beconfigured to transmit data to more than one controller.

An example communication cell in a cellular network comprises: aprocessing system comprising a controller and radio units, with theradio units being configured to communicate with the controller and tocommunicate wirelessly with mobile devices within the communicationcell. The processing system may be configured to perform operationscomprising: estimating signal strength experienced by the mobiledevices, with the signal strength being affected by interferenceexperienced by the mobile devices, and with the interference beingcaused by transmissions of at least some of the radio units withinranges of the mobile devices; and identifying, based at least on thesignal strength, two or more of the mobile devices that can be scheduledfor communication, on a same frequency, with two or more of the radiounits in the communication cell.

An example method is used in a cellular network comprising a processingsystem comprising a controller and radio units, with the radio unitsbeing configured to communicate wirelessly with the controller and tocommunicate with mobile devices within a communication cell. Theprocessing system performs operations comprising: estimating signalstrength experienced by the mobile devices, with the signal strengthbeing affected by interference experienced by the mobile devices, andwith the interference being caused by transmissions of at least some ofthe radio units within ranges of the mobile devices; and identifying,based at least on the signal strength, two or more of the mobile devicesthat can be scheduled for communication, on a same frequency, with twoor more of the radio units in the communication cell. The example methodmay be implemented using one or more non-transitory machine-readablestorage devices storing instructions that are executable to perform themethod.

An example communication system in incorporated into a cellular network.The communication system comprises a processing system comprising acontroller and remote units, where the remote units are configured tocommunicate with the controller and to communicate with mobile deviceswithin a communication cell of the cellular network. One or morenon-transitory machine-readable storage media store instructions thatare executable by the processing system to perform operationscomprising: estimating signal strength experienced by all or some of themobile devices; identifying, based at least on the signal strength, oneor more of the mobile devices that can be scheduled for communicationwith one or more of the remote units in the communication cell on a sameairlink resource; and scheduling the communication between the one ormore mobile devices and the one or more remote units.

An example communication system is incorporated into a cellular network.The communication system comprises a processing system comprising acontroller and remote units, where the remote units are configured tocommunicate with the controller and to communicate with mobile deviceswithin a communication cell of the cellular network. A method performedby the processing system comprises: estimating signal strengthexperienced by all or some of the mobile devices; identifying, based atleast on the signal strength, one or more of the mobile devices that canbe scheduled for communication with one or more of the remote units inthe communication cell on a same airlink resource; and scheduling thecommunication between the one or more mobile devices and the one or moreremote units.

An example communication system comprises: remote units to exchange RFsignals with mobile devices, with RF signals comprising informationdestined for, or originating from, a mobile device; and a controllercomprising a real-time scheduler for assigning airlink resources tomobile device for the information. The controller may be configured todetermine the remote unit transmission or reception needs of mobiledevices by estimating signal levels and representing the needs bynumerical values. The real-time scheduler may be configured to assignmobile devices to airlink resources, sometimes assigning two or moremobile devices to the same airlink resource according to RF isolation,based on the numerical values. The example communication system mayinclude one or more of the following features, either alone or incombination.

The remote unit transmission or reception needs may be determined basedon estimates of the signal loss between each of the remote units and themobile device. The remote unit transmission or receptions needs may bedetermined based also on the traffic load seen on each of the remoteunits. The signal loss may be estimated based on an uplink transmissionfrom the mobile device to the remote units. The uplink transmissions maycorrespond to Sounding Reference Signal (SRS) transmissions in the LTEstandard. The uplink transmissions may correspond to PRACH transmissionsin the LTE standard. The uplink transmissions may correspond to PUCCHtransmissions in the LTE standard. The uplink transmissions maycorrespond to PUSCH transmissions in the LTE standard. The numericalvalues may be derived from uplink measurements. The numerical values maybe binary taking the values 0 or 1. The numerical values may take onvalues from a finite number of levels greater than 2.

For each mobile device, the numerical values may be used to form aquantized signature vector. Operations for determining, based on thequantized signature vectors, that the two or more mobile devices can bescheduled on the same airlink resource for communication may comprisedetermining that the signature vectors are orthogonal. The quantizedsignature vector for a mobile device may be determined using a thresholdsignal-to-interference-plus noise ratio (SINR).

For each mobile device, the numerical values may be used to form aquantized signature vector and two users may be allowed to be scheduledon the same frequency resource when the sum of their quantized signaturevectors have no component that exceeds a preset threshold.

The numerical values may be determined based, at least in part, on thelocations of the mobile devices within the communication cell. Theassignment of two or more mobile devices on the same airlink resourcemay result in different remote units in the cell transmitting to adifferent mobile devices on the same airlink resource. The assignment oftwo or more users on the same airlink resource may result in some remoteunits not transmitting to any of the users. The assignment of two ormore users on the same airlink resource may result in some remote unitstransmitting simultaneously to multiple users. The remote units maytransmit simultaneously to multiple users have a reduced transmit power.

The controller may further reduce the transmission power to certainmobile devices that it determines to be near a remote unit. Thecontroller may make the determination based on measurements of uplinktransmissions of the mobile devices at the remote units. The uplinktransmissions may include LTE SRS, PUCCH, PRACH or PUSCH transmissions.

The operations may comprise determining bit rates at which data is to betransmitted to and from two or more mobile devices. Determining a bitrate for communication between a mobile device and a radio unit maycomprise: receiving, from remote units, information on measurements onan uplink control channel, and using such measurements in determiningthe bit rate. Determining the bit rate may include uncertainty due tosmall-scale fading.

Determining a bit rate for a communication from a remote unit to amobile device may comprise: receiving from the mobile device feedback onthe success or failure of past data transmissions, and using suchinformation in bit rate determination. The mobile device feedback may bean Hybrid ARQ (HARQ) feedback of LTE. The past HARQ feedback may beignored when the UE's dominant interferer has changed.

Operations for determining a bit rate for a communication from a remoteunit to a mobile device may comprise: receiving from mobile devicesmultiple channel state information (CSI) feedback, and using suchinformation in bit rate determination. Operations for determining a bitrate for a communication from a remote unit to a mobile devicecomprises: receiving from mobile devices multiple channel state feedbackincluding interference measurement, and using such information in bitrate determination. The interference measurement may be based on ChannelState Information Reference Signal (CSI-RS) of LTE. The mobile devicemay report multiple interference measurements for different interferencescenarios.

An example communication system comprises: remote units to exchange RFsignals with mobile devices, with RF signals comprising informationdestined for, or originating from, a mobile device; and a controllercomprising a real-time scheduler for assigning airlink resources tomobile devices for the information. The remote units are configured toperform at least some baseband processing, with the at least somebaseband processing including receiving and extracting the informationon the uplink control channel. Operations performed by the controllercomprise balancing the processing load across the remote units whenprocessing the uplink control channel, where load-balancing comprisessetting periods and phases for transmissions from some mobile devices soas not to overlap with transmissions of other mobile devices. Theexample communication system may comprise one or more of the followingfeatures, either alone or in combination.

The setting of periods and phases for uplink control channeltransmissions of a mobile device may also be based on the one or moreremote units that are processing the uplink control channeltransmissions. The setting of periods and phases for uplink controlchannel transmissions of a mobile device may be modified when a remoteunit processing the uplink control channel transmissions changes(because of mobility).

The uplink control channel transmissions may include Scheduling Request(SR) or Channel State Information (CSI) transmissions. The operationsmay comprise management of uplink control channel processing load on theremote units, where load management comprises adjusting periods fortransmissions from some mobile devices based on a communication trafficload in the communication cell. The communication traffic load may bemeasured based on the number of connected users.

The assigned same airlink resources may be for downlink transmissionsfrom the different remote units to the two or more mobile devices. Theassigned same airlink resources may be for uplink transmissions from twoor more mobile devices to remote units without substantial interference.The assigned same airlink resources may be for uplink transmissions fromtwo or more mobile devices to one or more remote units whose receivedsignals are jointly processed for reliable detection.

An example communication system comprises: remote units to exchange RFsignals with mobile devices, with RF signals comprising informationdestined for, or originating from, a mobile device; and two or morecontrollers comprising real-time schedulers for assigning airlinkresources to mobile device for the information. A coordination functionis coupled to the controllers to coordinate the assignments made by thereal-time schedulers in the controllers. The example communicationsystem may include one or more of the following features, either aloneor in combination.

The coordination function may reside in one of the controllers. Eachconnected user may be managed by one of the controllers. PRACHtransmissions may be demodulated and decoded by one remote unit, and thecontroller may be configured to manage a connected user is determined bythe one remote unit.

One or more of the controllers may act as a backhaul controller managingthe connection to the external network and upon receiving a page for amobile user it selects the controller to manage the mobile user. One ormore of the controllers may act as a timing source for the remote units.Two or more remote units may belong to the same cell and mobile devicesin the cell may be served by any one of the controllers via one or moreof the remote units. One or more of the controllers may provide the datafor the downlink common channels for the remote units.

At least some mobile devices may receive on two or more frequencycarriers, and each controller may handle one of the carriers, wherein amobile user is served by the controller associated with its primarycarrier.

The coordination function may be used to coordinate airlink resourceassignments across multiple frequency carriers. A remote unit canreceive data from more than one controller, and a remote unit cantransmit data to more than one controller.

Any two or more of the features described in this specification,including in this summary section, can be combined to formimplementations not specifically described herein.

The systems and techniques described herein, or portions thereof, can beimplemented as/controlled by a computer program product that includesinstructions that are stored on one or more non-transitorymachine-readable storage media, and that are executable on one or moreprocessing devices to control (e.g., coordinate) the operationsdescribed herein. The systems and techniques described herein, orportions thereof, can be implemented as an apparatus, method, orelectronic system that can include one or more processing devices andmemory to store executable instructions to implement various operations.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features andadvantages will be apparent from the description and drawings, and fromthe claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an example of a radio network.

FIGS. 2A and 2B are block diagrams showing an example of one cell of aradio network connected to a controller/control unit (CU) and two cellsconnected to a CU.

FIG. 2C is a schematic diagram of an example of a remote unit (RU).

FIG. 3 is a block diagram showing the deployment of an example radionetwork on a site, such as a building or other area.

FIGS. 4A-4C are block diagrams of examples of antenna mapping schemes ina cell.

FIG. 5A is a block diagram showing an example of virtual splitting in acell.

FIG. 5B is a block diagram showing an example of a controller detectingPhysical Random Access Channel (PRACH) transmissions.

FIGS. 6A and 6B are block diagrams of an example of a radio network withdifferent cell configurations at different times.

FIG. 7 is a block diagram showing examples of two resources grids fortwo corresponding antennas of a remote unit (RU).

FIG. 8 is a block diagram showing an example of signal transmissionsbetween user equipment (UE) and a remote unit (RU).

FIG. 9 is a block diagram showing an example of uplink compression.

FIG. 10 is a block diagram showing an example of side information on theuplink and the downlink between a controller (CU) and a remote unit(RU).

FIG. 11 is a block diagram showing an example of predictive quantizationfor the LTE Physical Uplink Shared Channel (PUSCH).

FIG. 12 is a diagram showing an example of subframe boundaries.

FIG. 13 is a diagram showing an example of downlink hybrid automaticrepeat request (HARQ) operation.

FIG. 14 is a diagram showing an example of subframe alignment.

FIGS. 15 and 16 are diagrams showing examples of HARQ timing for thedownlink and the uplink, respectively.

FIG. 17A is a block diagram showing an example of Soft Frequency Reuse(SFR) in LTE.

FIG. 17B is a block diagram showing an example of two cells implementingcoordinated scheduling.

FIG. 18 is a flow diagram showing an example of synchronization betweena controller and a remote unit.

FIG. 19 is a schematic diagram showing an example of a special subframeused in transitioning from DL (downlink) transmission to UL (uplink)transmission.

FIG. 20A to 20C are schematic diagrams showing examples of combiningsignals from different baseband modems at a controller and at the remoteunits, respectively.

FIG. 21A is a schematic block diagram showing an example of a subframe.

FIG. 21B is a schematic block diagram showing examples of subframes fortwo virtual cells.

FIG. 22 is a flow diagram showing an example of a process for groupingpredetermined RF zones offline.

FIG. 23 shows an example of a map determined offline for RF zones.

FIG. 24 shows a block diagram view of an example of a localizationprocess for pruning.

FIG. 25 is a block diagram showing the maintenance of a pruning set.

FIG. 26 is a block diagram showing an example of CU stacking in a cell.

FIG. 27 shows an example implementation showing communication betweenRU1, RU2, UE1 and UE2.

FIG. 28 is a block diagram showing examples of functionality splitsbetween RUs and CUs.

FIG. 29 is a block diagram showing an example topology for an exampleimplementation for use with the processes described herein.

FIG. 30 is a flowchart showing an example process for implementingautomatic CSI-RS configuration.

FIG. 31 is a block diagram showing an example of a clustered CSI-RSallocation.

FIG. 32 is a block diagram showing an example of a clustered CSI-RSallocation.

DETAILED DESCRIPTION

The systems and techniques described below are example implementationsof features that may be included in a radio access network. The claimsmade herein are not limited to the example implementations describedbelow.

Referring to FIG. 1, an example radio network 12 is deployed on a site10 so that one or more mobile operators, such as operator A 14 oroperator B 16, can provide mobile network access to one or more userequipment (UE(s)) 18, 20, such as smartphones, at site 10. The site maybe an enterprise or corporate building, a public venue, such as a hotel,hospital, university campus, or even an outdoor area such as a ski area,a stadium, or a densely-populated downtown area or a city. The radionetwork 12 includes controllers (each of which can also be referred as aController Unit (CU)) 22, 24 and Remote Units (RU) 26 a-26 i connectedby an Ethernet network 28. The CUs 22, 24 are connected (backhauled) tothe operator's core network, which may include a security gateway (SeGW)and nodes defined in the Long Term Evolution (LTE) standard, such asmobility management entity (MME) 14 a, 16 a and Serving Gateways (SGW)14 b, 16 b, optionally, through Home eNodeB gateways (HeNB GW) 30, 32.

The CUs may connect to the operator's core network via the Internet orother IP-based packet transport network 33 (for the purpose ofdiscussion, we may only refer to the network 33 as the Internet,although other networks may be used or included). When multiple CUs arepresent, one CU may act as an eNodeB termination point and present asingle network interface towards the core network; e.g., a SeGW, a MME,a HeNodeB GW or a SGW. CUs may be implemented using known principles ofNetwork Function Virtualization (NFV) as a virtualized softwareapplication running on a virtual machine/hypervisor. The virtualmachine/hypervisor may run on hardware that is shared with othervirtualized applications. The hardware may be an off-the-shelf ITserver. The CUs may also include certain MME functionality (not shown)and SGW functionality (not shown), thus allowing traffic to flowdirectly between the UE and a destination node 31 on the Internet or onlocal IP network 28 at site 10 without traversing the operator's corenetwork.

In some implementations, each CU 22, 24 performs the functions of a basestation, except for certain baseband modem and RF functions that may beperformed by the RUs. Each CU also may manage one or more of the RUs.Each CU may be associated with a mobile operator such that the RUs theymanage may operate on a spectrum that belongs to that mobile operator.It is also possible for a CU to be shared between multiple mobileoperators. Among other things, the CUs may schedule traffic to/from theUEs. Each CU 22, 24 is also connected to a service manager 40, 42, whichis typically located in the operator's core network. The service manageris responsible for the configuration, activation and monitoring of theradio network. There may also be a local facility service manager, whichcan allow local IT personnel to install and maintain the radio network.The RUs 26 a-26 i contain RF transceivers to transmit RF signals to andfrom the user equipment and to perform RF front-end functions, amongother functions.

Generally, a traditional base station, such as a traditional small cell,includes a Radio Frequency (RF) unit, a digital baseband modem unit anda network processing unit. Such a traditional base station implementsboth RF functionality and baseband processing. In some implementations,one or more traditional base stations can be in communication with acentralized controller. The baseband functionalities can be splitbetween the traditional base station and the centralized controller ofthe traditional base station(s) such that the centralized controllerperforms only the upper layer (e.g., Layer 3 or higher) processingfunctions of the baseband functionality.

In some implementations, the CUs do not perform any RF functions. EachCU can include one or more baseband modems, each for performingfunctions of all layers of baseband functionalities, including the MediaAccess Control (MAC) layer (Layer 2) processing, and upper layer (Layer3 and above) processing, as shown in configuration (a) of FIG. 28. Forexample, real-time scheduling, which is part of the MAC layer (Layer 2),may be performed by a baseband modem of a CU. Baseband modems may alsoperform physical layer (Layer 1) processing. In addition, the basebandmodems or the CUs may also perform other functions similar to thetraditional base station, such as the function of the network processingunit, e.g., processing Internet Protocol (IP) data.

In some implementations, real-time scheduling refers to assigning userdata to time and/or frequency resources based on CSI (Channel StateInformation). In downlink (DL) scheduling, CSI is supplied by the UE. Inthe LTE standard, the downlink CSI may include a Channel QualityIndicator (CQI), Precoding Matrix Indicator (PMI), or Rank Indicator(RI). In uplink (UL) scheduling, CSI is determined by the controllerbased on transmissions received from the UEs. The real-time schedulingis a Layer 2 function and is performed in the CU. In the LTE standard,uplink CSI may be determined based on the signals transmitted by the UE,for example, the Sounding Reference Signal (SRS). The baseband modemfunctions performed by the controller may also include Layer 1 functionssuch as downlink error control coding, uplink error control decoding,uplink multi-antenna diversity combining of signals received bydifferent RUs, channel estimation, and other higher layer functionsrelated to the wireless transmission or reception. In someimplementations all Layer 1 functions are implemented in the RUs, andonly the baseband functions of Layer 2 and above are implemented in theCUs, as shown in configuration (b) of FIG. 28. In some implementations,the Layer 1 functions (“Layer 1 (“Partial”)”) are split between the CUsand RUs, as shown in configuration (c) of FIG. 28. The uplink controlchannel receiver functions, such as PUCCH, PRACH, and SRS, may besubstantially implemented in the RUs, whereas the uplink PUSCH receiverfunctions can be handled by the CUs. The functional split between the CUand the RU may be different on the downlink and on the uplink. In someimplementations, substantially all downlink Layer 1 functions can beimplemented in the RUs and a majority of uplink Layer 1 functions can beimplemented in the CUs, as shown in configuration (d) of FIG. 28.

In some implementations, the CUs and the RUs of the network 12 performdistinctive functions in the radio network and are connected by Ethernetnetwork 28, although other transport networks, such as Hybrid Fiber-Coax(HFC) cable networks, VDSL (Very-high-bit-rate Digital Subscriber Line)networks, or wireless networks can also be utilized to enable thevarious capabilities described in this specification. The CUs 22, 24 maydetermine the processing capacity of the data/signal transmission atsite 10 for the functions implemented in the CUs, while the RUs 26 a-26i may provide RF/signal coverage to site 10, as well as the processingcapacity for the functions implemented in the RUs.

The CUs 22, 24 may contain one or more processors or other processingdevices on which code is executed to instruct performance of certainnetwork and baseband modem functions. The processors can be hardwareformed by Integrated Circuits (ICs) and other electrical components.Each CU 22, 24 may contain one or more baseband modem processors (see,e.g., FIGS. 2A and 2B) or may be configured to perform the functions ofone or more baseband modems. Each baseband modem may be implemented onone or multiple processors. When a baseband modem is implemented onmultiple processors, each processor may be responsible for processingsignals associated with selected groups of UEs. In some cases, the CUsmay be configured to perform no RF functionality. The RUs may becontrolled by the CUs and may be implemented by hardware blocks, such asradio transceivers (see, FIGS. 2A and 2B).

The RUs may have transmit antennas that are integral thereto or theantennas may be external, and connect to, the RUs via antenna cables. AnRU is also referred to as a radio point (RP), or a radio point unit(RPU). In some examples, there may be less software functionalityrunning on the RUs than on CUs 22, 24. In some implementations, the RUsare configured to perform no baseband modem functionality. In otherimplementations, the RUs may perform some baseband modem functionality.For example, in the LTE standard, the RUs may implement Fast FourierTransform (FFT) and Inverse FFT (IFFT) functions. In someimplementations, RUs may perform additional downlink baseband modemfunctions. For example, RUs may perform all, or the vast majority of,the Layer 1 functions. The baseband modems in the CUs and the RUs may beconnected through a standard off-the-shelf switched Ethernet network 28with one or more Ethernet switches 34, 36, 38, and possibly one or moreadditional switches in between switch 34 and switches 36, 38. In someimplementations, all CUs and RUs at site 10 are connected to each otherthrough the Ethernet network 28. Other networks may be used to connectthe CUs to the RUs, including wireless links, CATV networks or dedicatedfiber links.

In some implementations, one or more RUs, together with a baseband modemin a given CU, form a physical cell. In the example shown in FIG. 1, acell 44 includes RUs 26 a-26 d controlled by one or more baseband modems(not shown) in the CU 22, and a cell 46 includes RUs 26 e-26 icontrolled by one or more baseband modems (not shown) in the CU 24. TheRUs 26 a-26 i can be deployed at different locations of site 10, e.g.,different rooms, floors, buildings, etc., to provide an RF coverageacross the site as uniformly as possible. Each CU may have one or morebaseband modems and can control one or more cells. Nominally, eachbaseband modem may have the data transmission capacity of a single LTEsector, which can be quite large using frequency reuse techniquesdescribed in this specification. The number of baseband modems availableat the site and the capacity of each LTE cell typically determines thedata capacity that can be delivered to the site.

The radio network 12 of FIG. 1 can be implemented with various airinterface technologies. For example, 4G LTE may be used. LTE is astandard developed by 3GPP, a standards organization. The first versionof the LTE standard was made available in 3GPP Release (Rel.) 8.Subsequently, the LTE standard was refined in Releases 9, 10, 11 and 12.Several more releases of the standard will be developed in the future.3GPP Releases 8 to 11 of the LTE standard are used in the example radionetworks, systems, and methods described herein. However, the radionetworks and other systems and methods described herein can be utilizedwith any appropriate release of the LTE standard, includingFrequency-Division Duplex (FDD) and Time-Division Duplex (TDD) variants,or with a variety of other appropriate future (e.g., 5G) or existing airinterface technologies, such as the IEEE 802.11, which is more popularlyknown as Wi-Fi, or IEEE 802.16, which is also known as Wi-Max, or 3G airinterfaces such as Universal Mobile Telecommunications System (UMTS).

Commercial LTE networks may be synchronous such that timing phases ofall transmissions from eNodeBs are aligned with GPS (global positioningsystem) time or UTC (coordinated universal time). In a standalone LTEeNodeB, the GPS/UTC time is provided by a GPS receiver, which is aphysical component on the eNodeB hardware. In some implementations, thehardware of CUs 22, 24 include a physical GPS receiver to provide timingto the radio network 12. In deployments where CUs 22, 24 are distantfrom any satellite view, e.g., located deep inside a building, thephysical GPS receiver (not shown) can be external to the CU hardware andcan deliver the timing information to CUs 22, 24 through, e.g., theIEEE1588 PTP (precision time protocol). In some implementations, asource of timing for the radio network 12 is a timing server (not shown)located in the operator's network (e.g., the network 14, 16) thatprovides timing to CUs 22, 24 using, e.g., the IEEE1588 protocol. RUs 26a-26 i do not necessarily contain any GPS receiver in some cases, andmay receive timing information either from the CUs or directly from anexternal GPS receiver via IEEE1588 or other high-precision timingprotocols. Timing synchronization is discussed below.

Referring to FIG. 2A, in an example implementation, a CU 60 includes abaseband (cell) modem 62 connected to RUs 66 a-66 e through an Ethernetnetwork 68. RUs 66 a-66 e belong to the same cell 64. The positions ofthe RUs are chosen to provide RF coverage, which depends primarily onthe transmitter power of the RUs and the RF propagation environment atthe site. The data capacity of a single baseband modem can be shared byall UEs that are in the coverage area of the RUs that belong to thecorresponding cell. The number of RUs to be assigned to a single cellcan be determined based on the number of UEs in the coverage area of theRUs, the data capacity needs of each UE, as well as the available datacapacity of a single baseband modem, which in turn depends on thevarious capacity-enhancing features supported by the baseband modem.

In some implementations, in a radio network, the size and shape of thecells can be varied in a site according to the traffic demand. In hightraffic areas, cells can be made smaller than in low traffic areas. Whentraffic demand distribution across the site varies according totime-of-day or other factors, the size and shape of cells can also bevaried to adapt to those variations. For example, during the day, morecapacity can be delivered to the lobby areas of a hotel than to the roomareas, whereas at night more capacity can be delivered to the room areasthan to the lobby areas.

In some implementations, RUs 66 a-66 e can provide uniform signalstrength throughout the cell 64 without introducing any cell boundaries.When the capacity of a single baseband modem 62 is insufficient to servethe area, additional modems can be added to the CU or unused modems canbe enabled in the CU to split an existing cell into multiple cells. Morecapacity can be delivered with multiple cells. For example, as shown inFIG. 2B, a CU 80 includes modems 82, 84 controlling respective cells 86,88 through an Ethernet network 96. Each cell 86, 88 includes one or moreRUs 90 a, 90 b, 92 a, 92 b to provide RF coverage to UEs 94 a-94 d.Cells 86, 88 can be used by the subscribers of one mobile operator, orby different mobile operators. If needed, additional CUs with morebaseband modems can also be added. Additional RUs may be added to expandor improve the RF coverage.

In addition to the modems or modem functionalities, in someimplementations, CU 80 contains a coordination unit 98 that globallycoordinates the scheduling of transmission and reception of the modems82, 84 to reduce or eliminate possible interference between the cells86, 88. For example, the centralized coordination allows devices 94 c,94 d that are located within the overlapping boundary region 100 of thetwo cells 86, 88 to communicate without substantial inter-cellinterference. The details of the centralized coordination are discussedbelow. In some implementations, the interference issues that are likelyto take place in the boundary regions of multiple cells within theentire building or site may occur less frequently, because of therelatively few number of cells needed. In some implementations, theCU(s) can perform the centralized coordination for a relatively fewnumber of cells and avoid inter-cell interference. In someimplementations, coordination unit 98 may be used as an aggregationpoint for actual downlink data. This may be helpful for combiningdownlink traffic associated with different cells when multi-user MIMO isused between users served on different cells. The coordination unit mayalso be used as an aggregation point for traffic between different modemprocessors that belong to the same baseband modem.

Unless otherwise specified, the examples provided below are mostlydirected to one cell. However, the features described herein can bereadily extended to multiple cells. Referring to FIG. 2C, an example RU200 for use in the radio network of FIGS. 1 and 2A-2B can have twoantennas 202, 204 for transmitting RF signals. Each antenna 202, 204 maytransmit RF signals on one or more LTE channels (or carriers). The cellto which the RU 200 and its antennas 202, 204 belong has an ID(Cell-ID). The CU and its RUs and antennas may support multiple LTEchannels, each with a different Cell-ID. In addition, each antenna 202,204 is assigned to a unique Release 8 Cell-Specific Reference Signal(CS-RS) logical antenna port (ports 0, 1, 2 or 3) and, possibly, aunique Release 10 Channel State Information Reference Signal (CSI-RS)logical antenna port (ports 15, 16, . . . , 22). In this example,antennas 202, 204 are also referred to as physical antennas, while thelogical antenna ports are also referred to as virtual antenna ports. Inthe example shown in FIG. 2C, antenna 202 is assigned to the CS-RSlogical antenna port 0 and the CSI-RS logical antenna port 15; andantenna 204 is assigned to the CS-RS logical antenna port 1 and theCSI-RS logical antenna port 16. The logical antenna ports, together withthe Cell-ID and other parameters configured in the CU, determine theCS-RS (Cell-Specific Reference Signal) 206 the antennas transmit underRelease 8, or the CSI-RS (Channel State Information-Reference Signal)208 the antennas transmit under Release 10.

The RF signals transmitted by antennas 202, 204 carry LTEsynchronization signals PSS/SSS (Primary SynchronizationSignal/Secondary Synchronization Signal), which include a marker for theCell-ID. In use, an idling UE monitors the reference signals associatedwith a Cell-ID, which represents one LTE channel in one cell. Aconnected UE may transmit and receive RF signals on multiple LTEchannels based on channel aggregation, a feature of the LTE standardfirst defined in Release 10.

The RU 200 can also have more than two antennas, e.g., four, six, oreight antennas. In some implementations, all RUs in the radio network(e.g., the radio network 12 of FIG. 1) have the same number of transmitand receive antennas. In other implementations, the RUs have differentnumbers of transmit or receive antennas.

The radio networks described above can be upgraded in the CUs, e.g., tosupport future LTE or other standards, sometimes without makingsubstantial changes, e.g., any changes, to the deployed RUs. In someimplementations, when the RUs support multiple frequency channelssimultaneously, an upgrade for carrier aggregation can be performed byenabling additional channels in the same RU. Carrier aggregation canalso be implemented using RUs that operate on one selected carrier. Inthis regard, in some implementations, different single-carrier RUs canbe configured to operate on different carriers. RUs that operate ondifferent carriers need not be co-located. For example, in a simplelinear topology shown in FIG. 29, RUs operating on one carrier (CarrierA) (RU(A)) may be spatially offset relative to RUs operating on anothercarrier (Carrier B) (RU(B)). In some cases, this approach uses twocarriers to deliver a more consistent coverage by using one carrier tofill in the coverage edges for the other carrier. In more complextwo-dimensional or three-dimensional topologies, similar spatiallydistributed deployments can be used to bring a more uniform coverageacross two or more carriers and deliver a more consistent userexperience in some cases. In some cases, UEs may use different UplinkTiming Advance when operating on different carriers as described in LTERelease 11. In carrier aggregation using a single RU or multiple RUs,the aggregated channels may be in the same or different frequency bands.Likewise, when the RUs support frequency bands for the TDD(time-division duplex) version of the LTE standard, Time-Division(TD)-LTE capability may be added at a later date by upgrading the CU'sand possibly the RU's software/firmware, or by adding a new CU. If Wi-Fisupport is required, Wi-Fi capability may be added to the RUs. Wi-Fitransceivers in the RUs can be managed by the same or a differentcontroller and can be managed by the same service managers, both at thesite and in the operator's network. Such upgrades can, in some cases, beperformed in a cost effective manner, e.g., by making hardware changes(sometimes at most) in a relatively small number of CUs in a centrallocation (as opposed to replacing a large number of RUs that are spreadacross the site).

Radio Network Deployment

Referring to FIG. 3, an example radio network 120 is deployed at a site122. One or more CUs 124 are installed in a room 126, e.g., a telecomroom, locally at the site 122. The RUs 128 a-128 l are distributedaround the site 122. In some implementations, some RUs are wall-mountedwith integrated antennas, some RUs are hidden in one or more closets,and some RUs are installed above the ceiling tile and attach to awall-mount antenna via an external antenna cable.

In some implementations, the RUs 128 a-128 l connect to the CUs 124through a switched Ethernet network 130, which includes twisted pairand/or fiber optic cables, and one or more Ethernet switches 132.Components of the Ethernet network 130 may be standard off-the-shelfequipment available on the market. In some implementations, the Ethernetnetwork 130 is dedicated to the radio network alone. In otherimplementations, radio network 120 shares Ethernet network 130 withother local area traffic at the site 122. For example, in an enterprisenetwork such other traffic may include local traffic generated byvarious computers in the enterprise that may be connected to the sameEthernet switches. The radio network traffic can be segregated fromother traffic by forming a separate Virtual Local Area Network (VLAN)and high-priority QoS (Quality of Service) can be assigned to the VLANto control latency. In the example shown in FIG. 3, the CUs 124 areconnected to a co-located Ethernet switch 132 (in the same room 126). Insome implementations, the connection 134 uses a single 10 Gb/s Ethernetlink running over fiber optic or Category 5/6 twisted pair cable, ormultiple 1 Gb/s Ethernet links running over Category 5/6 twisted paircables.

Those RUs (not shown in FIG. 3) that are near the telecom room 126 maydirectly connect to the Ethernet switch 132 in the telecom room 126. Insome implementations, additional Ethernet switches 136, 138, 140 areplaced between the Ethernet switch 132 and the RUs 128 a-128 l, e.g., inwiring closets near the RUs. Each wiring closet can contain more thanone Ethernet switch (e.g., switch 136, 138, 140), and many Ethernetswitches can be placed in several wiring closets or other rooms spreadaround the site. In some implementations, a single Category 5/6 twistedpair cable is used between a RU and its nearest Ethernet switch (e.g.,between the RU 128 a and the Ethernet switch 136). The Ethernet switches136, 138, 140 may connect to the Ethernet switch 132 in the telecom room126 via one or more 1 Gb/s or 10 Gb/s Ethernet links running over fiberoptic or Category 6 twisted pair cables. In some implementations,multiple virtual RUs are integrated into a single physical device (notshown) to support multiple frequencies and possibly multiple mobileoperators. For example, an RU may support multiple carriers for carrieraggregation, the carriers may belong to different frequency bands,and/or some frequency bands may be unlicensed, as in LTE-Unlicensed(LTE-U).

Downlink Transmit Antenna Mapping in a Cell

Referring to FIG. 4A, an example cell 300 (controlled by a single modemor a single CU) contains sixteen RUs 302 a-302 p. N (an integer, e.g.,1, 2, 4, etc.) physical antennas of each RU may be mapped to a samegroup of CS-RS or CSI-RS virtual antenna ports 0 . . . N−1, as definedin the LTE standard. In the example shown in FIG. 4A, N is two, and themapping is done in the same manner as shown in FIG. 2C. In this exampleimplementation, all RUs 302 a-302 p in the cell 300 transmit the sameCell-ID on the same LTE channel, and all antennas share the same Cell-IDand broadcast the same Cell-ID in the Primary and SecondarySynchronization Signals (PSS/SSS). When an RU serves multiple channels,different channels may be using the same or different Cell-IDs. When aUE is located in the cell 300, the UE receives the reference signals ofthe same logical antenna port, e.g., port 0, from different physicalantennas of different RUs. To the UE, the RUs appear as part of a singlecell on a single LTE channel.

Alternatively, multiple RU clusters each containing one or more RUs maybe formed within a single cell. The antennas in the cluster may beassigned to different CS-RS or CSI-RS virtual antenna ports, but mayshare the same Cell-ID. For example, as shown in FIG. 4B, a cell 320contains 16 RUs 322 a-322 p each having two antennas and eight clusters324 a-324 h each containing two RUs. Within each cluster 324 a-324 h,the four physical antennas of the two neighboring RUs are assigned tofour different CS-RS virtual antenna ports 0, 1, 2 and 3 and/or to fourdifferent CSI-RS virtual antenna ports 15 through 18. As a result, acluster having a total of N (N is four in FIG. 4B) physical antennasappears to the user equipment as a single cell with N transmit antennaports.

Compared to the cell configuration shown in FIG. 4A, the number ofantenna ports seen by the user equipment is doubled in FIG. 4B. Theconfiguration of FIG. 4B can, in some cases, improve the performance ofthe UE, especially when the UE is near the coverage boundaries of two ormore neighboring RUs. Assuming that the UE has two antennas forreceiving signals, under Release 8, the UE can communicate with theradio network through 4×2 single-user MIMO (multiple-inputmultiple-output). In systems compatible with Releases 10-12 of the LTEstandard, up to four RUs with two transmit antennas each can be used toform an eight-antenna cluster, and then the UE can implement 8×2single-user MIMO. The same UE within a radio network having theconfiguration shown in FIG. 4A can communicate through 2×2 single-userMIMO. Even higher order MIMO communications, e.g., 4×4, 8×8, may beimplemented in some cases for UEs with four or eight receive antennas.

Increasing the number of physical transmit antennas involved in MIMOcommunications, e.g., using the configuration of FIG. 4B, may notsubstantially increase processing complexity, except (in some examples)when the number of layers in spatial multiplexing increases, e.g., from2 (FIG. 4A) to 4 (FIG. 4B). Although clusters of two RUs are shown anddiscussed, as explained above, a cluster can include other numbers ofRUs, and cell 320 can include clusters having different sizes.

In some implementations, a wrap-around structure is used by the CU inassigning the physical antennas to logical (or virtual) antenna ports,such that anywhere within the coverage of the cell 320, a UE can receivefrom as many logical antenna ports as possible. This wrap-aroundstructure can allow the single-user closed-loop MIMO to operate insidethe cell 320 seamlessly over a large coverage area.

Described below are examples of how CSI-RS (Channel State InformationReference Signal) can be used in the example systems described herein.In LTE, CSI-RS is a “cell-specific” pseudo-random reference signaltransmitted from 1, 2, 4 or 8 virtual antenna ports (or simply antennaports) on specific REs (resource elements) and subframes. In someimplementations, for 2, 4 or 8 antenna ports, a CSI-RS uses 2, 4 or 8REs (resources) per RB (resource block), respectively, and istransmitted in every RB across the entire transmission band periodicallyonce every P subframes. The CSI-RS period P can range from 5 to 80subframes in some examples. The mapping between CSI-RS virtual antennaports and physical antennas can be one-to-one or one-to-many.

In some examples, CSI-RS is used by the UE only for reporting CSI.Multiple CSI-RS may co-exist in a single cell or even the same RU. Insome implementations, each CSI-RS is defined by a) CSI-RS identity, b) anumber of antenna ports, c) a CSI-RS configuration index, whichindicates the position of the CSI-RS resources on a resource grid, andd) a subframe period and a relative offset. Different CSI-RS in the samecell may use different numbers of antenna ports, different periods,different CSI-RS configuration indices, and different mappings betweenantenna ports and physical antennas. As in CS-RS, the UE will assumethat all CSI-RS antenna ports are co-located. This means that in aCSI-RS system with more than two antenna ports or, more generally, whenthe CSI-RS antenna ports are not all mapped to physical antennas of thesame RU, the UE will not take into account differences in average pathloss or Doppler spread between antenna ports when reporting CSI.

In some implementations, CSI-RS (Channel State Information ReferenceSignal) is not typically (e.g., never) advertised by the eNodeB.Instead, in such implementations, active UEs are individuallyconfigured, during connection set-up, with one or more CSI-RS tomonitor. In some implementations, different UEs may monitor the same ordifferent CSI-RS. These different CSI-RS may have different number ofantenna ports, different subframe periods, or offsets, etc. A singlephysical antenna may transmit multiple distinct CSI-RS, although suchCSI-RS may need to be correctly configured to prevent interference insome cases.

In some implementations, simulcasting of CSI-RS, as implemented in thesystems described herein, uses two antenna ports that are pairwisemapped to physical antennas on RUs, as shown in FIG. 4A. In such atwo-antenna port CSI-RS, referred to herein as CSI_2, the two CSI-RSantenna ports 15 and 16 are mapped to physical antennas on RUs. In thisexample, each RU will be transmitting the exact same two-antenna portCSI-RS in a simulcast fashion. In another four-antenna port CSI-RS,referred to herein as CSI_4, the four antenna ports {15, 16} and {17,18} are mapped to physical antennas on pairs of RPs in an alternatingfashion, as shown in FIG. 4B. In this example, every pair of RUs will betransmitting the same CSI-RS from four physical antennas repeated acrossthe site in a simulcast fashion, but the transmissions from differentantenna ports are not all co-located. Differences in average path gain,Doppler spread, etc. between different antenna ports {15, 16} and {17,18} will not be accounted for by the UE. In distributed SU-MIMO (SingleUser Multiple-Input and Multiple-Output), gain imbalance can becompensated in the CU and/or the UE receiver. Gain compensation in theCU in the example systems described herein can be based on average pathloss measurements on the uplink. Similarly, a two-dimensional patterncan be created for an eight-antenna port resource, referred to herein asCSI_8.

In multi-RP CSI-RS configurations where different RPs transmit differentCSI-RS, example processes of assigning RUs or RU physical antennas toCSI-RS resources can be performed either manually or based onmeasurements of the uplink UE transmissions, for example SRStransmissions, at the RUs. The CU can use these UL measurements todetermine which RUs are neighbor RUs (in the RU topology) and forexample assign these RUs to the same CSI-RS cluster. Alternatively, insome examples, the assignment can be made based on radio environmentmonitoring provided by the RUs. REMs (Radio Access Maps or RadioEnvironment Maps) allow RUs to measure path gains between RUs, and theresulting information can be used in assigning RUs or RU physicalantennas to CSI-RS virtual antenna ports. A flow chart showing anexample process 3000 for automatic CSI-RS configuration is illustratedin FIG. 30. According to process 3000, a CU identifies (3001) one ormore neighboring RUs based on user equipment (UE) transmissions, such asLTE SRS. The CU forms (3002) clusters of RUs, and assigns RU antennas inthe clusters to CSI-RS virtual ports. The CU also determines (3003)CSI-RS configurations for all RUs taking into account UE transmissionson the uplinks, after which RUs begin transmitting according to thedefined configurations. The UEs are configured (3004) to report CSIbased on one or more of the CSI-RS.

In the examples above, there is a single CSI-RS, with 2, 4 or 8 antennaports, which (in this example) correspond to physical antennas on 1, 2or 4 RUs, reused across the entire site in a simulcast fashion. In someimplementations, once a UE is configured for one of these CSI-RS, thereshould not be a need to reconfigure the UE as it roams across a site.

In TM10 (Transmission Mode 10) of Release 11 of the LTE standard, a UEcan report multiple CSI. In the example systems described herein, theCSI configurations described below are designed to take advantage ofthis capability. Consider a clustered CSI-RS allocation as illustratedin the example of FIG. 31. In this example, different RU clusters 15-16are transmitting different CSI-RS. Specifically, the left-most 2 RUs3101 are assigned to the 2-antenna CSI-RS, CSI-2.0 3102, the next(middle) 2 RUs 3103 are assigned to 2-antenna port CSI-RS 2.1, 3104, andso forth. In this example, all antenna ports in a given CSI-RS areco-located. In this CSI configuration, when the UE crosses a certaincluster boundary, a CSI reconfiguration becomes necessary.

In a clustered configurations, in some implementations, in order toavoid interference between CSI-RS and PDSCH, zero-power CSI-RS may betransmitted on REs that correspond to a neighbor cluster's CSI-RS. Italso may be necessary, in some cases, to configure the UE with thesezero-power CSI-RS. This informs the UEs of the positions of REs wherethey should not expect PDSCH. Such zero CSI configurations may not beneeded for CSI configurations transmitted by distant CSI clusters.

Open Loop Power Control in a Single Cell with Multiple RUs

In the LTE standard, a UE estimates the UL path loss based on the DLpath loss. This is known as Open Loop Power Control (OLPC) and is usedto set the initial transmit power of the UE in random access forconnection establishment. The DL path loss is estimated from themeasured RSRP (Received Signal Reference Power) and the known CS-RStransmit power which is advertised by the eNodeB. In some examples, itis sometimes necessary to transmit CS-RS at different power levels fromdifferent RUs. Since eNodeB advertises only one value for CS-RS transmitpower, and since the UE has no ability to distinguish CS-RStransmissions from different RUs, an alternate method may be used formore accurate open loop power control. In future versions of the LTEstandard this can be achieved using a flexible signal such as CSI-RS,where different RUs or at least different clusters of RUs can transmit auniquely distinguishable CSI-RS reference signal. In order to preventinterference between CSI-RS and PDSCH transmissions, zero-power CSI-RStransmissions may also be used in neighboring RUs or RU clusters. Thetransmit power level and configuration of each CSI-RS can then beadvertised. Additional power offsets can be advertised to account forpossible uplink combining. The UE will measure the received power levelfor all advertised CSI-RS resources, select the strongest CSI-RS or thestrongest few CSI-RS and determine its own UL transmit power level forPRACH accordingly.

Downlink Simulcast and Coordinated Transmission

Referring again to FIGS. 4A and 4B, in this example, all antennasassigned to the same logical (or virtual) antenna port transmit the samereference signals (CS-RS or CSI-RS) in a time-synchronized manner. Insome examples, the assignment can reduce the effects of shadow fadingthrough macro diversity. The assignment can also present a multipathchannel to each UE (not shown). A UE may report a single CSI feedback(including CQI (channel quality indicator) and PMI/RI (pre-coding matrixindicator/rank indicator)) based on the CS-RS or CSI-RS referencesignals it receives from all transmitting antenna ports in the cell.When physical antennas of different RUs are transmitting the samereference signal, in some cases the UE may experience richer scatteringand a more MIMO-friendly Rayleigh-like channel without significantinterference from other transmit antennas in the same cell. Furthermore,the UE only sees one physical cell, and there is no need for any handoffwhen the UE is in the coverage area of multiple RUs that belong to thesame physical cell.

A single broadcast channel PBCH (physical broadcast channel) is used inexample cell 300 or example cell 320. The cells 300, 320 also implementa single downlink control region for transmitting signals on PDCCH(physical downlink control channel), PHICH (physical hybrid-ARQ(automatic repeat request) indicator channel) and PCFICH (physicalcontrol format indicator channel). Other common logical channels, suchas the paging channel PCCH (paging control channel) that are transmittedover PDSCH (physical downlink shared channel) may also be shared.

As discussed previously, in an example, all physical antennas that areassigned to the same logical or virtual antenna ports, such as theRelease 8 CS-RS logical antenna ports and the Release 10 CSI-RS logicalantenna ports, transmit the same control signals and reference signals.In the example shown in FIG. 4B, all PDCCH/PHICH/PCFICH transmissionsuse 4-antenna TX (transmit) diversity and all transmissions from thoseantennas assigned to the same logical antenna port are identical. A UEwithin the cell 320 perceives transmissions from those antennas assignedto the same antenna port as if the transmissions are delivered from asingle antenna through a multipath channel.

Furthermore, in some implementations, capabilities in Release 11 can beused to improve downlink MIMO operation inside a large cell, such ascells 300, 320, that has many RUs. In Release 11, multiple non-zeroCSI-RS resources can be used inside a single cell. As an example,referring to FIG. 4C, each RU 402 a-402 p (or clusters of RUs) of a cell400 is assigned to a different CSI-RS resource with a distinct CSIscrambling ID 404 a-404 p. Each RU (or RU cluster) with the distinct CSIscrambling ID operates as if it were a virtual cell, even though theyshare the same physical Cell-ID with other RUs in the same cell. Themultiple CSI-RS resources (and scrambling IDs) in the cell 400 aremonitored by the UE. In some implementations, the UE can be configuredby the CU (not shown, e.g., the CU 22, 24 of FIG. 1) of the radionetwork to perform the monitoring of multiple CSI-RS resources.

A UE (not shown) in the cell 400 sends multiple CSI reports to the CU ofthe radio network for multiple RUs whose CSI-RS transmissions the UEmonitors. From each CSI report, the CU obtains a CQI for the respectiveRU(s) and uses the CQI for determining signal strength from that RU. TheCU can use these multiple CQI reports along with multiple PMI/RI(Pre-coding Matrix Indicator/Rank Indicator) reports received from theUE to determine precoder coefficients. Furthermore, Release 11 supportsenhanced CQI reporting based on accurate interference measurements bythe UE. Release 11 also includes an E-PDCCH (enhanced physical downlinkcontrol channel), which can be used to increase the control channelcapacity in the cell 400. Features of Release 11, such as thosedescribed above, may be used to enhance the functionality of the systemsdescribed herein.

In some implementations where the radio network supports multiple cells,downlink transmissions in different cells can be coordinated to reduceinterference. Coordination may be achieved using techniques such as Hardand Soft Frequency Reuse (HFR/SFR) or Release 11 Coordinated Multipoint(CoMP), which are described below

LTE Unlicensed

In some implementations, carrier aggregation across licensed andunlicensed frequency bands (or, simply, “bands”) can be used. An exampleof such a system is LTE-Unlicensed (LTE-U). In LTE-U, there is a primarycarrier that operates on an operator's licensed band and one or moresecondary carriers that operate over unlicensed band, such as the 5 GHzISM band. In some implementations, the primary carrier is used to handlethe UEs mobility and all radio resource management for the UE. In someimplementations, each RU simultaneously supports both licensed andunlicensed carriers. In some implementations, LTE-U is implemented onlyin the downlink. In some implementations, multiple RUs may transmit thesame Physical Cell-ID on the same primary carrier and present a singlecell to the UE on the primary carrier, thereby avoiding handovers. But,the same RUs may also operate on one or more additional secondarycarriers on the unlicensed bands. RUs that operate on additionalsecondary carriers may transmit different Physical Cell-IDs on thesesecondary carriers. In this case, the UEs can be configured to sendmeasurement reports based on RSRP and RSRQ (Reference Signal ReceivedQuality) measurements on these secondary carriers. Such measurementreports can be used by the controller in coordinated scheduling. In someimplementations, a single controller can manage both licensed andunlicensed carriers. Different functional splits can be used on thelicensed and unlicensed carriers. For example, on the uplink, all Layer1 processing can be performed in the RUs on the unlicensed band and/orat least some Layer 1 processing can be performed in the CU. On thedownlink (DL), RUs may perform some or all of the Layer 1 processing.

Uplink Diversity Reception

The uplink transmissions by a UE that is being served by a cell withmultiple remote units may be received by all the RX (receive) antennasin these RUs. When the UE is near the coverage boundaries of two or moreRUs, its transmissions may be received by RX (receive) antennas of theseRUs. In this situation, the uplink performance can be improved byperforming diversity combining (e.g., Maximal Ratio Combining (MRC),Interference Rejection Combining (IRC) or Successive InterferenceCancellation (SIC) in the controller) across signals received bymultiple RUs. By having multiple RUs send received IQ data to thecontroller, multi-antenna/multi-RU combining can be achieved.

When there are two or more cells in the radio network, uplinktransmissions of a UE that is being served by a first cell may bereceived by the RX antennas of one or more RUs that belong to othercells. In this situation, uplink performance can also be improved byperforming diversity combining (e.g., MRC, IRC or SIC) across signalsreceived by multiple RUs, including the RUs that belong to differentcells.

There may be different options for implementing the uplink combiningfunction described above. For example, the uplink combining can beperformed entirely in the CU. In this example, the RUs forward, to theCU, at least some compressed IQ data, and the CU performs the combiningoperation(s) (e.g., executes instructions to perform the combining).Alternatively, the RUs may fully, or partially, decode the signalsreceived via their own RX (receive) antennas, and send the decoded dataand/or certain soft decision metrics (e.g., quality metrics) to the CU,where the final combining can be performed.

Virtual Cell Splitting

The capacity in the radio network can be increased by a cell splittingprocesses. In an example process, RUs in a single cell are split betweentwo cells, increasing the capacity at the site. The two cells candeliver up to twice the capacity because two UEs can be served in twodifferent cells on the same time-frequency resource.

Alternatively, the capacity of a single cell can be increased by usingvirtual cell splitting. The cells each containing multiple RUs asdiscussed above can be virtually split, by allowing multiple UEs totransmit simultaneously using the same time-frequency resources, usingeither multi-user MIMO, which is an extension of single-user MIMO tomultiple UEs supported in the LTE standard, or RF isolation. In contrastto real cell splitting, in some implementations, virtual cell splittingdoes not impact the reference signals or common control channels.Virtual cell splitting may increase cell capacity by allowing multipleUEs to transmit or receive data using the same time frequency resources.

Downlink Virtual Cell Splitting

Multi-User MIMO

In some examples, virtual cell splitting is implemented with multi-userMIMO, which is used to send data to multiple UEs on the same PDSCHtime-frequency resource. The multiple UEs can be served on the sametime-frequency resource even when these UEs receive strong RF signalsfrom the same antennas.

In multi-user MIMO, a unique set of precoder weights is applied tomodulation symbols destined to each UE to prevent interference betweenco-scheduled UEs. For example, when each UE has a single antenna,individually generalized beams are formed for each UE. When each UE hasmultiple antennas, the CU and the RUs may provide spatial multiplexing(e.g., sending multiple layers of modulation symbols) to each UE, inaddition to serving the multiple UEs on the same time-frequencyresource.

Multi-user MIMO can be used with the antenna mapping schemes shown inFIGS. 4A and 4B. For example, in the antenna mapping scheme of FIG. 4A,two UEs can be served on the same time-frequency resource by one or moreRUs. The CU for the cell 300 forms two beams in directions of thestrongest RF paths for the two UEs, without causing significantinterference between the two UEs.

In Release 8, multi-user MIMO is supported in downlink transmission mode5. Each UE having a single antenna reports to the CU a 2×1 precodingvector selected from a 4-entry precoding codebook and an associated CQI,which is based on single-user beam forming using the selected precodingvector. When the precoding vectors selected by two UEs are orthogonal toeach other, the CU may schedule the two UEs on the same time-frequencyresource using half of the available transmit energy for each UE.

For two UEs that have no inter-user interference cancellationcapabilities, the multi-user MIMO with the antenna mapping technique ofFIG. 4A may not introduce substantial interference when each UE receivesdownlink signals from both antennas of a RU at about the same strength,and when the selected precoding vectors of the two UEs are orthogonal toeach other.

Multi-user MIMO can also be implemented with advanced UEs that arecapable of using knowledge about the modulation structure of interferingsignals from co-scheduled UEs to reduce interference. In someimplementations, a UE with two or more antennas can remove part of theinterference using spatial filtering.

In Transmission Mode (“TM”) 8 or 9 of Release 9 or 10, multi-user MIMOcan be implemented using DM-RS (demodulation reference signal), whichmay allow the CU to use any appropriate precoder without being limitedto those precoders that are defined in the standard in so-calledcodebooks. The UE reports the CSI to the CU implicitly by selecting aprecoder from a predetermined codebook along with a Channel QualityIndication (CQI). In some implementations, the UE determines the CSIusing the CSI-RS reference signal, which can support up to 8 antennaports. In one example, the same CSI-RS signal is transmitted from allphysical antennas of the RUs that are assigned to the same CSI-RSlogical antenna port and the UE reports only one CSI (e.g., CQI/PMI/RI)for each (physical) cell. In Transmission Mode 9, the CU can schedule upto 4 UEs on the same time-frequency resource with up to 2 layers per UEand up to 4 layers per RB (Resource Block). The CU transmits DM-RS(Demodulation Reference Signal) on 12 REs (Resource Elements) per RB andthe 12 REs are used for all UEs that are co-scheduled on the sameresource. The transmission based on DM-RS can provide flexibility andsimplification in scheduling.

In some implementations, when the CU knows the channel coefficients, theCU chooses the precoding vectors for the UEs to provide each UE with themaximum SINR (Signal-to-Interference and Noise Ratio) without the UEexperiencing substantial interference. As discussed previously,interference suppression capabilities provided by the UEs can furtherfacilitate reliable multi-user MIMO.

Release 11 supports using multiple CSI-RS signals inside a singlephysical cell and allows a UE to send more than one CQI/PMI/RI reportper physical cell. This can improve the performance of the multi-userMIMO. For example, in Release 11, each RU (or each group of RUs) may beassigned to a CSI-RS reference signal that is different from thoseassigned to the other RUs in the cell, or at least in some part of thecell. Each UE is requested to report multiple CSI individually formultiple RUs in the cell. The CQI/PMI/RI information obtained from themultiple reports can be more accurate than information obtained from asingle report. Based on the accurate information, the CU can determinewith greater precision the precoding vectors in multi-user MIMO andreduce inter-user interference. In some implementations, the CUconfigures each UE with a selected set of CSI-RS, e.g., but notnecessarily the entire set, of CSI-RS resources available in the cell sothat the UE does not have to send CSI reports for all CSI-RS resourcesin the cell.

RF Isolation

Virtual cell splitting in a cell can also be achieved based on RFisolation among the UEs in the cell. Virtual cell splitting with RFisolation differs from multi-user MIMO based virtual cell splitting inthat transmissions from an RU are not generated using a joint precodingoperation on symbols representing data for multiple UEs. In some cases,the transmissions from an RU represent data of one UE. In someimplementations, the transmission from an RU may represent data frommultiple UEs, for example UE1, UE2 and UE3, but then such transmissionis not generated using a joint precoding operation on symbolsrepresenting data from all the UEs, UE1, UE2 and UE3.

In some implementations, multiple UEs are served simultaneously on thesame time-frequency resource via RUs or antennas whose coverage areas donot substantially overlap. For a first UE, instead of simulcasting thesame PDSCH signal on all physical antennas that are assigned to the samevirtual antenna port, only some RUs and physical antennas are allowed totransmit the signals to the first UE. Transmissions from other RUs andphysical antennas to the first UE are purged. One or more of the RUsthat are not transmitting to the first UE can instead transmit to asecond UE on the same time-frequency resource. When the transmissionsfrom the physical antennas of the RUs serving the first UE are receivedat a relatively (e.g., very) low level by the second UE, and likewisewhen the transmissions from the physical antennas of the RUs serving thesecond UE are received at a relatively (e.g., very) low level by thefirst UE, no significant interference occurs, even when the UEs do nothave any interference suppression capabilities. This may be due to theirspatial separation.

When UE is configured for a transmission mode that supports DM-RSreference signals, DM-RS are transmitted similarly to the PDSCH signals.For example, the DM-RS reference signals for the first UE may betransmitted only from the antennas of the RUs that are serving the firstUE. In Release 10, multi-user MIMO can be used to send up to four layersto two or more UEs. In some implementations, additional operations mayneed to be implemented to reduce or avoid interference between UEs. Inthe example shown in FIG. 5A, two UEs 502, 506 at different locations ina single cell 500 are co-scheduled on the same time-frequency resourcebased on RF isolation with up to two layers per UE. The cell 500includes 12 RUs 506 a-506 l, each having two physical antennas andtransmitting CSI-RS on virtual antenna ports 15 and 16. To serve two UEsthat are spatially far apart in a given subframe, the single cell 500 isvirtually split to form three virtual cells 508 a, 508 b, 508 c. The RUs506 a, 506 b, 506 g, 506 h in the virtual cell 508 a serve the userequipment 502. The RUs 506 e, 506 f, 506 k, 506 l in the virtual cell508 c serve the user equipment 506. The RUs 506 c, 506 d, 506 i, 506 jin the virtual cell 508 b do not serve any UE in order to avoid causinginterference to the UEs 502 and 506. The total number of layersco-scheduled in the single cell 500 is four. The virtual cells describedabove are not static like physical cells. The virtual cells can varydynamically from one subframe to the next and across resource blocks. Insome implementations, the dynamic variation applies only to the shareddata channel PDSCH. For example, there may be no virtual cell splittingin one subframe, while in another subframe, two different virtual cellsplitting may be applied in two different groups of resource blocks. Insome implementations, a virtual cell may have a single RU withoutsimulcasting. The virtual cells represent the ability of the system toserve multiple UEs in the same cell on the same time-frequency resource.

The RUs within the same virtual cell transmit the same DM-RS referencesignal selected, e.g., from four available ports/scrambling index {7.0,7.1, 8.0, 8.1}. The virtual cells that are located adjacent to eachother (or close to each other without directly bordering each other),such as the virtual cells 508 a, 508 b and the virtual cells 508 b, 508c, may use different DM-RS port numbers. Those virtual cells that arerelatively far apart, e.g., the virtual cells 508 a, 508 c, can reusethe same DM-RS reference signal based on the RF isolation. In suchimplementations, signal transmissions between the UEs and the radionetwork are performed without significant interference between thevirtual cells.

In some implementations, the CSI-RS configurations that take advantageof LTE Release 11 Transmission Mode 10 can be used. As describedearlier, these CSI-RS configurations are designed to utilize multipleCSI reports from the UE.

In some implementations, the CU chooses a MCS (Modulation and CodingScheme) for each co-scheduled UE based on the CQI values, determined bythe UE from the CS-RS or CSI-RS signals, reported by the UE. The CS-RSor CSI-RS signals are transmitted continuously by all physical antennasin the physical cell, including some antennas that may, at times, nottransmit on the shared data channel PDSCH. The same CS-RS or CSI-RSsignals transmitted from the physical antennas that are near the UE,when received at sufficiently high strength, are seen by the UE asmultiple transmission paths, or RF multipath. In some implementations,the UE can predict a higher (or lower) CQI based on the multipath thanthe actual CQI the UE will experience when receiving on PDSCH with lessmultipath. In such implementations, the HARQ (hybrid automatic repeatrequest) capability in the LTE standard can provide dynamic adaptabilityto reduce the effect caused by the mismatch between the predicted CQIand the actual CQI. In some implementations, when the actual channelconditions are worse than the conditions predicted by the CQI, the CUretransmits the data or signals with incremental redundancy to achievethe maximum data rate that the channel can support. In implementationsthat use Transmission Modes 9 or 10, the CSI-RS configurations can bechosen to facilitate UE measurement of interference, and in some modes,multiple CSI-RS configurations can be used for the UE to report CSIunder different interference conditions. The real-time scheduler in theCU can use such reports in choosing a MCS taking into account othertransmissions in other virtual cells on the same time-frequencyresource.

Resource Block Reuse

In each cell, RUs transmit data, e.g., user data or control data, todifferent UEs at each transmission time interval (TTI), e.g., of 1millisecond. An example time-frequency resource grid 2100 for LTEtransmission is shown in FIG. 21A, where the vertical axis representsfrequency and the horizontal axis represents time. A new resource gridis sent at each TTI of 1 millisecond. The discussion below uses 1millisecond only as an example and can be generalized to any other TTIs.In some implementations, each resource block is transmitted, typicallyat a set of contiguous frequencies different from the frequencies of theother resource blocks. As a result, in some implementations, eachresource block can serve one UE without interference from transmissionson the same resource block to other UEs. However, the capacity of thecell may be limited by the size of the resource grid 2100, whichincludes 50 resource blocks 2102 in 10 MHz LTE.

As described above, the capacity of the cell can be increased bytransmitting to multiple UEs on the same resource block. The differentUEs served using the same resource block can be viewed as belonging todifferent virtual cells. As a result, at a given TTI, those UEs thatneed to receive data from the RUs of a cell on certain RBs are groupedinto multiple virtual cells. In some implementations, the UEs indifferent virtual cells can be served on the same resource block, suchas the resource block 2102 in resource grid 2100 of FIG. 21A, withoutsignificant interference between them. In the example shown in FIG. 21B,two UEs in different virtual cells use resource blocks 2102 a and 2102 bin two virtual replicas 2100 a, 2100 b of the same resource grid in thesame TTI. In some implementations, each virtual cell at a resource blockhas one UE assigned to that resource block. The UEs in a physical cellcan be grouped into more than two virtual cells in a resource block sothat more than two UEs may share the same resource block. The use of thevirtual cells, or equivalently reusing the same resource block formultiple UEs can increase a cell's capacity. The UEs in differentvirtual cells on the same resource block can be served with lowinterference between them and multiple UEs can be scheduled dynamicallyin different virtual cells on the same resource block in a scalablemanner, e.g., two virtual cells, three virtual cells, and etc.Typically, in a cell, the number of UEs that need data transmission in agiven TTI can be 50, 60, or more, e.g., 100 to 200.

In some implementations, for data transmission in each TTI, a scheduler(e.g., a real-time scheduler) in a controller of the cell is configuredto (1) select UEs to be assigned to the same resource block. Theselection and the assignment may be performed such that transmission ofdata to different UEs on the same frequency resource interferes witheach other as little as possible. The scheduler is also configured to(2) select transmission strategies based on the assignment of the UEs.For example, the scheduler determines which RU(s) serve which UEs.Furthermore, the scheduler is configured to select the data rate for thedata transmission to each scheduled UE. In other words, the schedulerdetermines the number of bits that can be sent to each UE in theresource blocks assigned to that UE. Generally in LTE, the data rate toa UE depends on the SINR the UE is experiencing.

A similar coordinated scheduling exists when a centralized scheduler(e.g., one or more computer programs running in one or more CUs) isscheduling users across multiple physical cells. The example processesdescribed below can also be utilized when scheduling users acrossmultiple physical cells in, at, or via a coordinated centralizedscheduler.

UE Assignment

To perform task (1) above, at each TTI, the scheduler uses a signaturevector for each active UE. In some implementations, all RUs in the cellare instructed to listen to the transmission of each active UE in thecell to determine uplink average path gains p_(kj), where j representsthe j^(th) RU in the cell and k represents the k'th active UE. In theLTE standard, such uplink measurements can be based on SRS, PRACH oreven other UL (uplink) transmissions such as PUCCH or PUSCH. In general,the path gain p_(kj) between a UE and an RU on the uplink issubstantially equal to the path gain between the same RU and the UE onthe downlink. The signature vector of the k'th active UE can beexpressed asp _(k)=(p _(k1) ,p _(k2) , . . . ,p _(kJ))^(T),where J is the total number of RUs in the cell.

Given two UEs with signature vectors p_(k) and p_(l), the quality ofpairwise reuse in which the two UEs are assigned to the same resourceblock, can be estimated based on total interference I seen by the twoUEs:I(p _(k) ,p _(l))=1^(T) m(p _(k) ,p _(l)),where 1^(T)=(1, 1, . . . , 1), and m(p_(k), p_(l))=[min(p_(k1), p_(l1)),min(p_(k2), p_(l2)), . . . , min(p_(kJ), p_(lJ))]. As used herein,“reuse” includes, but is not limited to, two devices in a single cellutilizing the same resource (e.g., frequency) for communication withinthat cell. The “reusing” device may be the remote units (RUs), the userequipment (UEs) (e.g., a mobile device), or any other appropriatedevice.

Using the signature vectors of the active UEs, RF zones can be createdwithin the cell. Each RF zone represents a physical zone in which UEshave similar signature vectors such that, if these UEs are served on thesame resource block, the interference among these UEs will exceed apredetermined threshold. In some implementations, each active UE belongsto one and only one RF zone. UEs in different RF zones may be assignedto be served using the same resource block. The total number of RF zonesto be created can be predetermined, e.g., based on predicted needs, ordetermined dynamically based on real-time needs. For example, there canbe J times n RF zones for a cell, where n is a positive integer and J isthe number of RUs. In some implementations, each RF zone has an areathat is smaller than the total coverage provided by a single RU. Forexample, when a cell has 16 RUs, there can be 128 RF zones.

In some implementations, the RF zones of a cell are determined offline.The assignment of the active UEs to the predetermined RF zones can beperformed in real time in a computationally efficient manner. For apredetermined number of RF zones, each zone is identified by a zonesignature vector each identified by a single signature vector:z _(m)=(z _(m1) ,z _(m2) , . . . ,z _(mJ)),where m represents the m^(th) RF zone, and J is the total number of RUsin the cell. The signature vector of an RF zone can be mathematicallyrepresented as the centroid of all the signatures of all UEs that maybelong to that RF zone. RF zones and their signature vectors depend onthe number of RUs J in the cell. For a given value of J, thepre-determined RF zones can be kept fixed (fixed RF zones). In someimplementations, the RF zones can be modified (adaptive RF zones) duringactive use based on actual UE signature vectors encountered at a givendeployment.

The active UEs are grouped into the different RF zones in real timebased on one or more criteria. For example, a UE may be grouped into anRF zone that has the shortest Euclidean distance between the signaturevector of the UE and the RF zone. In some implementations, alocalization module in the controller keeps track of the RF zones foreach RRC (Radio Resource Control)-connected UE. The UEs in the same RFzone have signature vectors that produce a relatively large interferencemetric (e.g., greater than a threshold) among the UEs and between eachUE and the signature vector of the RF zone.

The UEs in different RF zones may be assigned to the same resource blockbased on their respective RF zones' signature vectors. In other words,the UEs assigned to the same resource block are determined by groupingdifferent RF zones. Each zone grouping may be associated with acorresponding reuse metric, such as the improvement in the transmissionrate for example. In general, RF zones that are ‘near’ each other arenot good candidates for reuse partnership. Based on the reuse metric,groups of RF zones that are good candidates for reuse can be determinedoffline. This zone grouping is performed for many levels of reuse. Forexample, for reuse level 2, an ordered list of all allowable pairings ofzones is created, in descending order of reuse metric. Alternately, eachpairing could also be tagged with a reuse metric in addition to theordering. Similarly for reuse level 3, all allowable triplets of zonesare computed and listed in descending order, with or without anassociated reuse metric. Other methods of organizing the RF zonegrouping table is also possible, such as hierarchical ordering. FIG. 23shows an example of such zone grouping. The grouping table of FIG. 23has a tree-like structure, which is traversed to identify UEs tocommunicate on the same frequency in a single cell without resulting insignificant interference.

The decision to assign UEs to the same resource block can then be madebased on the RF zone to which the UEs belong. The location of the UEwithin the RF zone need not affect the UE's assignment to a resourceblock.

An example process 2200 for grouping predetermined RF zones offline isshown in FIG. 22. Initially, an RF zone x is selected (2202). The RFzone x corresponds to a first RF zone. Next, M₁ number of candidate“pairable” RF zones are considered (2204) to be assigned to the sameresource block. M₁ can be 1, 2, . . . , up to M−1, where M is the totalnumber of RF zones. The best candidate RF zone with low (e.g., less thana specified) interference is first selected. Then additional RF zoneswith low interference to RF zone x are considered (2206). If no pairableRF zone is found, then the process 2200 ends (2210). If a pairable RFzone is found, additional M₂ candidate RF zones are considered for 3-waygrouping (2208) and determination is made (2212) whether they can begrouped with all previously selected RF zones. If no, then the process2200 ends (2210). If yes, then the process continues with the selectionstep 2208 for 4-way grouping, with the value of i increased by 1. In theabove procedure, the process progresses one-step at a time, making finalRF zone selections at each operation in the flowchart. Alternatively,the process may consider multiple hypotheses for RF zones at eachoperation, and make a final selection after examining all hypothesisover multiple operations.

In some implementations, the controller (or scheduler implemented by thecontroller or controllers) assigns UEs into different resource blocks inreal time in each TTI. The UEs are assigned to resource blocks based onQuantized Signature Vectors (QSVs). For example, at the beginning ofeach TTI, the controller uses a quantized version of a signature vectorp_(k)=(p_(k1), p_(k2), . . . p_(kJ))^(T) for each connected UE k. Thecalculation of the signature vector is described above. In someimplementations, the quantized signature vector p_(k) can be determinedbased on a predetermined path gain threshold p_(threshold), which is apositive number no greater than one.

For example, the controller quantizes the signature vector by settingall p_(ki) that are less than p_(threshold) times p_(k_max) to be zeroand all p_(ki) that are equal to or are greater than p_(threshold) timesp_(k_max) to be 1. In this example, p_(k_max) is the maximum path gainin the UE's path gain vector, typically the path gain to the closest RU.

In some implementations, a goal of assigning the UEs to resource blocksand serving certain UEs on the same resource blocks is to provide a goodtradeoff between capacity and fairness and to improve uneven userexperiences within the cell with a relatively low (e.g., minimal)reduction in capacity. Generally, a single one in the quantizedsignature vector with no other ones indicates a strong signal beingreceived from the closest RU relative to the other RUs. A “1” indicatesa preference for the UE or a UE need to receive its data (as opposed toanother UE's interfering data) from the corresponding RU. Multiple onesin a signature vector may indicate that the UE receives relativelystrong signals from corresponding multiple RUs and, therefore wouldprefer to receive its own data from these RUs rather than interferingdata. Possibly, the UE is in between two RUs and prefers to receivesignal from both RUs. A “0” in a signature vector indicates the UEreceives a relatively weak signal from a corresponding RU, and thereforecan tolerate receiving interfering data being sent to another UE fromthat RU.

In some implementations, two UEs can be assigned to the same resourceblock only if their quantized signature vectors are orthogonal to eachother. Orthogonality may be determined by performing a logical “AND”operation between two signature vectors, with two vectors beingorthogonal when each element of the result is a logical “0”.

The choice of the threshold p_(threshold) can determine what isconsidered to be a sufficiently strong signal to reserve a particular RUfor a UE. If the threshold is chosen to be very high, quantizedsignature vectors will typically have only one “1” corresponding to thenearest RU. This means that two UEs' quantized signature vectors will beorthogonal except when the two UEs have the same RU as the RU with thehighest path gain (e.g., same serving RU). This means that the schedulerwill allow any two UEs to be scheduled on the same frequency as long asthey have different serving RUs. This may lead to excess interferenceand low throughput for UEs located in between two RUs. If the thresholdis chosen to be too low, then UEs will have many “1s” in their signaturevectors and as a result very few UEs can be scheduled to transmit on thesame resource block. This avoids interference between UEs, but resultsin a low capacity of the cell. A low threshold can be appropriate whenthe instantaneous demand is low. When the load is higher, a largerthreshold may be preferred. In general, having a “1” in a quantizedsignature vector corresponding to an RU adds traffic load to that RU.Therefore, more quantized signature vectors are allowed to have 1s wherethe RUs are deemed to have low load, and fewer quantized signaturevectors are allowed to have “1” where the RUs are deemed to have highload. The determination of “low” or “high” load at an RU can be based onresource block utilization at the RU in recent TTI. The threshold valueused at each RP may then vary according to the recent load seen at a RP.

The controller can also determine demand based, e.g., on the amount ofdata waiting in buffers for transmission. The threshold can also bedetermined empirically based on other factors and other appropriateavailable channel information. For example, the UE may measure thedownlink signal strength and quality based on receive CS-RS referencesignal transmissions, and report the results of these measurements tothe controller in a measurement report in the form of RSRP or RSRQvalues. These RSRP and RSRQ values represent average signal strengthseen by the UE from CS-RS transmissions in nearby physical cells. RSRPand RSRQ represents the aggregate signal strength and quality from alltransmissions from RUs in the same physical cell. The controller can usethis information in selecting the quantized signature vectors. UEs mayalso send instantaneous channel quality indication (CQI) to thecontroller, in some cases at a much faster rate. CQI measurementsrepresent the instantaneous signal quality seen by the UE in CS-RS orCSI-RS transmissions received from the serving cell. In examples wheremultiple RUs belong to the same cell, the CQI report will correspond tothe aggregate channel from all transmissions from RUs in the same cell.

In some implementations, the quantized signature vectors may be chosenbased on some estimate of the average SINR of the UE. In someimplementations, the quantized signature vectors may be determined basedon an expected data rate of a user and how this rate changes as more“1s” are added to the quantized signature vector.

In some implementations, a quantized signature vector p_(k-quan) onlycontains zeros and ones. As an example, assuming that one example cellcontains 12 RUs and three active (or connected) UEs, three active UEscan have the following quantized signature vectors:

-   -   UE1: p₁=(110000000000);    -   UE2: p₂=(000011000100);    -   UE3: p₃=(011100000000).

In this example, p₁ and p₂ are orthogonal (since logically “AND”ing thetwo vectors results in all 0's). Accordingly, UE1 and UE2 can beassigned to the same resource block. p₃ is orthogonal to p₂ but not top₁. Accordingly, UE2 and UE3 can be assigned to the same resource block.However, UE1 and UE3 are not orthogonal and therefore cannot be assignedto the same resource block in this example implementation. In someimplementations, the threshold p_(threshold) is determined based on therequired SINR (signal-to-interference-plus-noise ratio) for transmittingdata to the UE. Accordingly, in such implementations, all RUs thatcorrespond to a one in the quantized signature vector have to transmitthat UE's data in order for the UE to receive the data at the requiredSINR. In other implementations, p_(threshold) may vary across RUs andmay depend on the recent load of the RU.

This example with three UEs can be extended into any appropriate numberof UEs in a cell. Following quantization of the signature vector foreach active UE, the UEs are assigned to resource blocks based on theorthogonality of groups of active UEs. Effectively, an “AND” Booleanoperator is applied to the group of active UEs. If the result of theoperation is all zero, then the UEs can share the same resource block.If the result of the operation contains any ones, then the UEs areserved on different resource blocks. In some implementations, the “AND”operator is applied pairwise among the signatures of all of the UEs. Ifeach of the pairwise-ANDs is 0, then all UEs can share a resource block.If not, at least one UE must use a different resource block. In order toallow the same UE to be assigned to different resource blocks andgrouped with a different set of UEs in each resource block, thecontroller can represent a UE in multiple instances depending on itsdata demand. Each instance corresponds to a single resource block or aresource block group. In some implementations, a resource block groupincludes an integer number of resource blocks. In some implementations,a resource block group will include three resource blocks. The same UEcan be grouped with different sets of UEs on different resource blocksor resource block groups. In some implementations, the processesdescribed herein only allow UEs with orthogonal quantized signaturevectors to share the same resource block or resource block group.

In some examples, Quantized Signature Vectors (QSVs) can take-onnon-binary values (values other than 0 and 1). Non-binary values can beused in QSVs to represent a blanking need of a UE from an RU. In someimplementations, a blanking need is a need for a UE operating in atransmission mode based on user-specific reference signals to receive nosignal from a UE. Non-binary values can also be used in QSVs to allowmulti-user transmission to more than 1 UE from an RU. The scheduler inthe controller may decide which users can be scheduled on the sameresource based on processes (e.g., mathematical calculations) other thantesting for orthogonality. For example, the scheduler can determine theeligibility of two UEs for sharing the same resource block by comparingthe sum of the QSVs and testing whether any of the components of the sumQSV exceed a certain threshold.

In some implementations, the QSVs can be regarded as vectors havingnumerical values that represent the transmission (on the downlink) orreception (on the uplink) need of a UE from an RU. For example, a valueof “1” may indicate UEs need to receive its own data from an RU.Alternatively, a value of “0” may indicate that the UE can receiveinterference from that RU and, therefore, that RU can transmit toanother UE. Other values, for example, 0.001, may be chosen to indicatethat the UE needs blanked transmission or multi-user transmission fromthe corresponding RU. In blanked transmission, the RU does not transmitany signal. In multi-user transmission, the RU transmits to two or moreUEs at the same time, often at a lower power level, as described below.When a value such as 0.001 is used, the real-time scheduler can assigntwo users to the same resource block when the sum of their QSVs has nocomponent greater than 1. Specifically, two UEs can be assigned to thesame resource block when they both need blanked transmission from an RU.

RU Assignment

In addition to assigning active UEs to resource blocks for datatransmission in each TTI, the controller (or the real-time scheduler)may also need to determine which RU of the cell transmits to whichactive UE on each resource block in the TTI. Without reuse, on eachresource block, all RUs that belong to the same physical cell transmitto one UE and the effective channel the UE experiences is a sum of theindividual channels from each of the RUs in the cell and the receivingantenna(s) of the UE. Accordingly, when the UE uses cell-specificreference signals transmitted by all RUs to estimate the transmissionchannel, the estimated channel is the same as the actual channel.However, when there is reuse and two or more UEs are being served on thesame resource block, the effective channel the UE experiences can beslightly different from the channel the UE measures from the CS-RSreference signals. This mismatch between the actual channel and theestimated channel is sometimes called bias. The transmission between aUE and the RUs can be improved by taking into account both this bias anddirect interference. Generally speaking, for a group of UEs sharing thesame resource block, a given UE receives its own data from the RPscorresponding to “1”s in the UE's signature vector.

When the QSVs use numerical values other than 0 or 1, on a givenresource block the RUs that correspond to positions in the sum QSV withvalues between 0 and 1 (e.g., 0.001) use either blanking (silent) ormulti-user transmission as described further below.

The positions of the 1's in each UE's signature vector p_(k) correspondsto the minimum set of RPs that will be transmitting to that UE. Inaddition, the transmission strategy for RPs which are 0's in both (orall) the UE's signature vectors in some resource block can be decided ina few different ways, as described below.

In some implementations, an RU may only transmit to the active UE thathas the strongest path gain to that RU. For example, when one valuep_(kj) for the kth UE is relatively large compared to path gains toother UEs, then it may be most efficient for the j^(th) RU toexclusively serve the k^(th) UE. Sometimes two active UEs sharing thesame resource block may have similar path gains to an RU. In suchsituations, overall interference can be reduced by having the RUtransmit to two or more UEs on the same resource block and at the sametime.

In general, the total interference seen by K UEs in reuse from an RU ona given resource block can be reduced (e.g., minimized) by scaling downthe transmit power of the RU by a factor P_reduction and allocatingpower to each UE in proportion to their path gain. In an example,P_reduction can be written as:P_reduction=(1/K)(u _(j_rms) /u _(j_mean))²,where u_(j_mean) is the mean path gain and u_(j_rms) is the RMS pathgain of the K UEs in reuse relative to that j'th RU. For example, whentwo UEs are in reuse (K=2) and they have the same path gain to an RU,P_reduction is equal to 0.5 (e.g., power cut in half). The RU dividesthe resulting 50% power equally between the two UEs. The total bias plusdirect interference seen by the two UEs is then reduced by 50%, whencompared to the case where the RU is transmitting full power to one UE.

Another way of reducing the interference seen by a UE from an RU is toblank transmissions from that RU. This may be helpful in avoidinginterference to other macro cell or small cell networks or when usingcertain transmission modes in LTE that utilize DM-RS or user-specificreference signals, examples of which include, but are not limited to,TM8, TM9, and TM10.

Yet another way of reducing interference seen by a UE is to reduce thepower of transmissions from an RU when the RU is serving a nearby UE. Insome implementations, the controller can keep track of the position ofthe UEs and, when a UE is determined to be near an RU, the transmissionpower to that UE is reduced. Reducing the transmit power when servingnearby UEs can significantly reduce interference to other usersreceiving on the same resource block at the same time, especially forUEs that utilize DM-RS of the LTE standard for channel estimation. Insome examples, to avoid any bias in the channel estimate of a UE thatutilize CS-RS reference signal of the LTE standard for channelestimation during demodulation, the relative power of the PDSCHtransmission relative to CS-RS transmissions to the UE is reduced aftersending a radio resource control (RRC) reconfiguration message. When thesame UE moves away from the RU, its power is increased again by sendinga new RRC reconfiguration message.

When a UE receives data from two or more RUs, it is possible to servethat UE using distributed higher-order MIMO across such RUs. Forexample, when two RUs, each with two transmit antennas are transmittingto a UE, instead of simulcasting, one can use precoding across the fourantennas. If the UE has two receive antennas, 4×2 distributed MIMO canbe implemented, using a 4×2 precoding matrix. The 4×2 precodingoperation can be implemented in the controller or the precodingoperation can be broken-down into two separate 2×2 precoding operationsand implemented in the RUs. The RUs may transmit suitable CSI-RSsignals, as described herein, and the UE, upon configuration by thecontroller, will report CSI based on these CSI-RS signals. Thecontroller will use the reported CSI to determine the precoder.

Likewise, it is also possible to use the methods described herein fordistributed multi-user MIMO. Using a method similar to that describedfor multi-user transmission from an RU, it is possible to schedulemultiple UEs on the same RB, when their CSI reports indicate that theyare candidates for multi-user MIMO transmission. When testing forpossible reuse, two UEs that can be in multi-user MIMO, one or more “1”positions in the UEs quantized signature vectors can be replaced by avalue between 0 and 1. This will allow these UEs to be reuse usingmulti-user MIMO.

Link Adaptation

In addition to determining resource blocks, the RU(s), and precodingstrategies for use in transmitting data to a UE, in someimplementations, the controller (or the real-time scheduler) alsodetermines the airlink transmission rate to the UE. In an example,suppose UE1 and UE2 are scheduled to reuse the same resource block, andthe controller receives s CQI (channel quality indicator) from both.However, the CQI reported by the UE does not consider possibleinterference that may be caused by the reuse. To successfully transmitdata to a UE at the correct transmission rate, the interference level isestimated and the actual transmission rate is determined based on theCQI and the measured interference level. The inclusion of the estimatedinterference in obtaining the correct transmission rate is alsosometimes called CQI backoff. Similarly in MIMO transmission modes, therank or precoder of the transmission could be different from the rankthat the UE requested, and this also may be determined based on abackoff mechanism that incorporates the estimated interference.

CQI backoff can be implemented based on path gain measurements on theuplink. On a TD-LTE system, downlink and uplink transmissions occur onthe same frequency and therefore result in similar channel coefficients.This reciprocity can be used to accurately estimate the downlinkinterference conditions based on measurements of uplink transmissionsfrom the UE. In an FDD system, downlink and uplink transmissions occuron different frequencies and, as a result, it is more difficult toestimate downlink interference condition based on uplink measurements.When estimates of downlink interference is limited to average path gain,then the rate estimate can take into account possible increase ininterference due to small-scale fading. By applying additional back-offreliable transmission can be achieved even in the absence of preciseinformation about the downlink interference.

In addition, the controller receives feedback from the UEs about thesuccess or failure of previously scheduled PDSCH transmissions as partof the HARQ process. The controller can further use this feedbackinformation for outer-loop link adaptation to adapt the transmissionrate to make the transmissions more reliable in some cases. What may, insome cases, limit the effectiveness of outer-loop link adaptation is therapidly varying interference conditions caused by independent schedulingin uncoordinated base stations. In the example systems described herein,outer-loop adaptation can operate in coordination with user schedulingand QSV selection modules, such that significant short-term changes inthe interference environment can be avoided in some cases. A UEsouter-loop is said to be in active state when the UE has been allocatedresources within the last N ms, where N is a configurable numericalparameter. Interference variation can be avoided, in some cases, byensuring that the UE's dominant interferer is persistent while the UEremains in active state. When the UE's dominant interferer is allowed tochange, e.g., due to uncertainty of the small-scale fading experiencedby the new dominant interferer, there may be some uncertainty in thehighest achievable rate of the UE and, therefore, a more conservativerate may be used. In some implementations, a stable dominantinterference environment can be achieved using “dummy” transmissionsfrom otherwise unused resource blocks in an RU that acts as a dominantinterferer. Similarly, QSV adjustments based on load changes that resultin a reduction of the simulcast zone of a UE may be deferred until theUE's outer loop transitions into the idle state. When a change in a UE'sQSV that reduces its simulcast zone is needed to relieve load in aneighboring RU, the UE's outer loop can be re-initialized even when theUE's outer-loop is in active state. Further, an airlink scheduler cantake advantage of variations in reuse between users to provide somefrequency diversity against varying interference.

Using the Interference Measurement (IM) capabilities in LTE TM 10/11described earlier in the clustered configuration of CSI-RS, linkadaptation performance can be improved. For example, a UE can beconfigured to report CSI based on multiple interference scenarios, andthe CQI and PMI can be determined based on such reports by the CU usingprecise knowledge about the user scheduling across the site. MultipleCSI reports by a UE can also be used to aid link adaptation.

PDSCH Reuse in Release 10/11 Transmission Modes 9/10

In Transmission Mode 9 of Release 10 and Transmission Mode 10 in Release11, DM-RS is used for equalization and demodulation. For each RB(resource block), there are two reference sequences, each using adifferent one of two scrambler identities. Furthermore, two orthogonalDM-RS sequences are derived from mapping a single QPSK referencesequence to 12 REs per RB using a length-2 orthogonal cover. Theselected DM-RS sequence is indicated on the PDCCH.

In some examples, different RUs may transmit to different UEs on thesame RB as follows: one MIMO layer each can be transmitted to two UEswith DM-RS sent on twelve REs per RB using an orthogonal cover. Theinterference between the two transmissions to the two UEs is small ornon-existing. One MIMO layer each can be transmitted to four UEs withDM-RS on twelve REs per RB, using a scrambling identity in addition tothe orthogonal cover. In this case, there is no interference betweenDM-RS transmissions to a first pair of UEs, UE₁ and UE₂, or betweenDM-RS transmissions to a second pair of UEs, UE₃ and UE₄. However, theremay be random interference between the two pairs. In someimplementations, two MIMO layers each are transmitted to two UEs, withDM-RS on twelve REs per RB, with orthogonal transmission between layersof the same UE, and random interference between the UEs. When reuseneeds more than four MIMO layers in total, the same DM-RS can be reused.

For example, to transmit two MIMO layers each to three UEs on the sameRBs, the same two DM-RS sequences having the same orthogonal cover andscrambling identity can be used for two UEs that are furthest apart. Forthe third UE, a DM-RS sequence having the same orthogonal cover as thetwo DM-RS sequences, but a different scrambling identity, can be used.

Rel. 11, TM10 of the LTE standard includes the capability for a UE toreport CSI for up to three CSI-RS. For example in the clusteredconfiguration described herein, a UE may be configured to report CSI notonly for its serving cluster, but also for the neighboring cluster.

Rel. 11, TM10 of the LTE standard includes the capability for the UE tomeasure interference coming from certain RUs and to include suchmeasurement in CQI estimation. To trigger interference measurement, a UEcan be configured with a CSI-IM (Channel-State Information-InterferenceMeasurement) configuration. The resources (REs) that the UE uses tomeasure interference may be defined in the same manner as in a CSI-RSconfiguration; e.g., a CSI-IM resource configuration index and a CSI-IMsubframe configuration index. In an example, one CSI-RS, together withone CSI-IM, forms a CSI process, which corresponds to one CSI report bythe UE. In Rel. 11, a UE can be with configured with at most four CSIprocesses (e.g., four CSI reports), three non-zero power CSI-RS, andthree CSI-IM.

In the above example, a TM10 UE may be configured with up to threeCSI-IM, with each CSI-IM corresponding to a different one of the RUclusters. By selecting RU clusters that a) are most likely to be used inreuse for that UE, and b) represent areas where the interferencemeasurement is most useful for that UE, one can control the number ofinterference measurements the UE needs to perform and report. In somecases, interference measurement may be most useful when the interferenceis expected to be strong and cause significant CQI back-off, asdescribed elsewhere herein. An RU cluster that is too close, or too faraway from, the UE may not be a good candidate for interferencemeasurement. In some examples, the UE is configured with one CSI-RS,three CSI-IM, and up to four CSI processes, where three processes areformed by pairing the UE's CSI-RS (“serving cluster”) with each of thethree CSI-IM, and a fourth process is formed by using the UE's CSI-RSwithout any CSI-IM.

In the example described above, cluster size can be varied from 1 to N(where N is a number greater than one) RUs. In some implementations,larger clusters may reduce the frequency of reconfiguration, but mayalso make it harder to match actual reuse conditions. It is alsopossible to form clusters with four or eight antenna ports. In someexamples, such clusters support CSI for distributed multi-RU MIMO.

Clusters can also be overlapping in some cases. For example, a slidingwindow of clusters can be defined, each of which may be represented by adifferent CSI-RS, as shown in the linear topology of RUs below. Thisoverlapping configuration may avoid cluster boundaries and, in someexamples, ensure that a UE is centrally positioned relative to its“serving” cluster. Overlapping clusters may, in some examples, increaseoverhead, since multiple CSI-RS may need to be transmitted from each RU.In the example of FIG. 32, RU2 through RU5 are transmitting threedifferent CSI-RS and RU1 and RU6 are transmitting two different CSI-RS.More specifically, in system 3200 of FIG. 32, RU1 is transmittingCSI-2.1 and CSI-2.12 and RU2 is transmitting CSI-2.12 and CSI-2.2 (wheretransmission in FIG. 32 is indicated by the dashed lines), and so forthfor the other RUs shown. When a UE is located in between RU1 and RU2,the UE can be configured to report CSI based on CSI-2.12, or somecombination of CSI-2.12, CSI-2.1 and CSI-2.2. The CU can use suchreported CSI to determine whether to serve the UE from RU1, from RU2, orfrom both.

The systems described herein may also use additional CSI-IMconfigurations for interference measurement. In this example, a CSI-IMconfiguration represents a certain interference scenario, where some RUsare transmitting and others are silent. For example, a UE receiving datafrom RU3 and RU4 may receive interference from RU2 or RU5 or both. Tofacilitate proper interference measurement, the CU can configure RU2 andRU5 with Zero Power CSI-RS, ZP-CSI-2.2 and ZP-CSI-2.5. Three CSI-IMresources can then be defined as follows:

CSI-IM Resource A: CSI-2.2 ZP-CSI-2.5 CSI-IM Resource B: ZP-CSI-2.2CSI-2.5 CSI-IM Resource C: ZP-CSI-2.2 ZP-CSI-2.5

The UE can be configured to report CSI on three CSI processes, whichinclude CSI resource CSI-2.34 paired with three different CSI-IMresources (A, B and C). Equipped with three distinct CSI reports thatcorrespond to these three CSI processes, A coordinated scheduler(running, e.g., in one or more CUs) can perform relatively accurate linkadaptation to determine the data rate to apply to the UE.

PDSCH Reuse in Carrier Aggregation

The techniques described above can also be used in conjunction withcarrier aggregation (CA). In CA, the real-time scheduler (e.g., in oneor more of the CUs) can schedule two or more users on two or morecarriers simultaneously. When the carriers are co-located, the same QSVscan be used to schedule on both carriers. In other words, UEs that canbe assigned to the same airlink resource on one carrier can also beassigned on the same airlink resource on other carriers.

When the carriers are not co-located (in other words, some RUs in acluster serve different carriers), then UE localization may be repeatedfor the two carriers and different QSVs may be needed on differentcarriers.

PDCCH Reuse

In a single-cell system with no re-use, PDCCH is simulcast and there isno interference. In some implementations, in a single-cell system withno inter-cell interference, fewer CCEs (control channel elements) can beused per DCI (downlink control information) than in a multi-cell system,thereby increasing capacity of the single cell. When a single-cellsystem is required to schedule transmission of many more UEs and nospare CCEs are available, in that same subframe PDCCH reuse can beimplemented. The implementation can be similar to the PDSCH reusedescribed above. The set of CCEs allocated to one UE can also be reusedfor another UE. Such reuse can increase control channel capacity of thesingle-cell system. LTE Release 11 provides increased PDCCH capacitywith a new E-PDCCH (enhanced PDCCH) channel. PDCCH reuse is linked withPUCCH reuse on the uplink, which is described below. The PUCCH resourcefor HARQ ACK/NACK from a given UE is identified by the CCE index thatwas assigned to that UE over the PDCCH. In PDCCH reuse, if the same setof CCEs were reused, two PDCCH transmissions start on the same CCEnumber and therefore, the corresponding PUCCH transmissions by the twoUEs will use the same PUCCH resource for HARQ ACK/NAK. Resource reuse inPUCCH transmissions creates correlated inter-UE interference, becauseboth UEs use the same DM-RS sequence. More complex blind decoders can beused to increase the PUCCH decoder reliability in such reuse scenarios.

The virtual cell splitting techniques described herein using multi-userMIMO or RF isolation can be utilized in systems that are compatible withall Releases of the LTE standard. Release 8 UEs use CS-RS, instead ofDM-RS, for demodulation, which in some situations, may cause mismatchduring demodulation. Still in many cases, virtual cell splitting in themanner described herein may be desirable, e.g., when there is a strongRF isolation between the transmitting and the non-transmitting antennassuch that the UEs can achieve total throughput higher than when eitherUE is served on a dedicated time-frequency resource.

In Releases 9 and 10, in some implementations, the single CQI/PMI/RIfeedback sent by the UEs may not be sufficient for the CU to determinereliably which RUs and physical antennas are most likely to provide thestrongest signal to each UE (in the downlink direction). In suchimplementations, the CU can also use information about the strength ofuplink signals, such as the Sounding Reference Signal (SRS) or PUCCHcontrol signals or PUSCH uplink data, received by the RUs from the UEsto determine the antennas that are likely to provide the strongestsignal to each UE on the downlink. After the CU determines the RUs orphysical antennas for transmission to a given UE, the CU chooses theprecoding vector weights as described earlier in the document so thatsignals to a UE are transmitted from antennas that the UE hearsstrongly.

In some cases, the virtual cell splitting using RF isolation can beimplemented with higher accuracy in Release 11, where the UEs arecapable of sending multiple CQI reports for different RUs. The CU usesthese CQI reports to determine which RUs or physical antennas transmitsignals that are likely to be received by co-scheduled UEs at a highstrength.

Uplink Virtual Cell Splitting

Referring again to FIG. 5A, virtual cell splitting may also beimplemented on the uplink. The CU may schedule multiple UEs on the sametime-frequency resource and reduce or remove any interference betweenco-scheduled UEs in the CU using Interference Rejection Combining (IRC),Joint Detection (JD) or Successive Interference Cancellation (SIC).These techniques can rely upon spatial filtering as in multi-user MIMOor as in RF isolation. On the uplink, the UEs 502, 504, 506 sharecertain uplink resources that are available in the cell 500. The uplinkresources can include the cyclic shift for DM-RS reference signals andthe Orthogonal Cover Code (OCC) that are assigned to UEs for PUSCH(Physical Uplink Shared CHannel) transmissions and the resource indicesassigned to UEs for PUCCH (Physical Uplink Control CHannel)transmissions. The CU can create virtual cells on the uplink by reusingthe same resources among UEs in the same physical cell. The number ofUEs that can simultaneously transmit on the same time-frequency resourcemay be limited at least partially by the availability of the uplinkresources in the single cell. Reusing the same resources among UEs canincrease the total capacity available on the uplink.

PUSCH Transmissions

The DM-RS reference signals used by a UE depend on the number ofResource Blocks (RBs) assigned to that UE. For PUSCH transmissions, thenumber of RBs can be as high as 108. A DM-RS reference signal having alength of 12×N is derived from a base sequence of the same length, whereN is the number of RBs assigned to the UE. Up to 12 DM-RS referencesequences (or interchangeably, signals) can be derived from each basesequence using a cyclic shift in the time domain. Thesecyclically-shifted reference sequences are orthogonal to each other.When the channel for transmitting the reference sequences issufficiently flat across one RB, two UEs can transmit their DM-RSreference signals with different cyclic shifts on the same RB. The CUcan then estimate respective uplink channels for the transmissions fromthe two UEs without experiencing any substantial interference betweenthem. When the channel is not sufficiently flat, in some cases, fewerthan 12 orthogonal DM-RS reference sequences can be generated bycyclically shifting a base sequence.

In some implementations, the orthogonal DM-RS reference sequences areused for single-user spatial multiplexing (up to 4 layers) andmulti-user MIMO. In Release 10, an orthogonal cover code can be appliedto the two DM-RS sequences such that two layers can be transmitted usingthe same cyclic shift, while keeping the DM-RS reference signalsorthogonal.

In some implementations, the UEs that are served by the same physicalcell (e.g., cell 500 of FIG. 5A) use the same base sequence for PUSCHtransmissions. When multiple UEs transmit on the same time-frequencyresource, the CU coordinates the assignment of cyclic shifts and theorthogonal covers in uplink scheduling to keep the DM-RS referencesignals transmitted on the same time-frequency resource orthogonal, whenpossible. In some cases, such orthogonality requires not only orthogonalDM-RS, but also perfectly aligned RB allocations. When UEs in reuse haveorthogonal DM-RS, but their RB allocations are not perfectly aligned,there may be some random DM-RS interference between the UEs. However,the performance impact of such random interference is small.

In some implementations, a sufficient number of cyclic shifts remainavailable for assignment and for use in spatial multiplexing ormulti-user MIMO in each cell. For example, when six cyclic shifts of thebase sequence are available and the six cyclic shifts are coupled with apairwise orthogonal cover code, the CU can serve as many as twelvelayers on the same uplink time-frequency resource with orthogonal DM-RSreference signals.

In some implementations, a physical cell described previously (e.g., thesingle cell 500 of FIG. 5A) can be arbitrarily large. In a large cell,when there is extensive use of simultaneous uplink transmissions on thesame time-frequency resource, the CU may be short of available cyclicshifts and orthogonal covers to maintain the orthogonality among theDM-RS reference signals. Similar to the RF isolation on the downlink,the uplink can reuse the one or more DM-RS reference signals on the sametime-frequency resource when the uplink transmissions by theco-scheduled respective UEs do not substantially interfere with eachother. In some implementations, when there is no substantial overlapbetween signals received from the co-scheduled UEs by certain groups ofRUs or receive antennas, the same DM-RS reference signal can be used forthose UEs. The CU can determine which groups of receive antennas or RUsare receiving significant signals from a UE based on PUCCH, SRS(Sounding Reference Signals) and prior PUSCH transmissions, and canassign cyclic shifts and OCCs (Orthogonal Cover Codes) accordingly.

In some implementations, when there are multiple cells served by one ormore controllers, it is also possible to assign the same base sequenceto all cells. This allows the controller to assign all UEs to cyclicshifts of the same base sequence and to ensure orthogonality betweenUEs, including those UEs that are served by different cells. Based onthe RF isolation, the controller can also reuse the same cyclic shiftsin different parts of the site and increase the number of UEs that canbe supported.

In a radio network compatible with the Release 11 standards, differentRUs in a cell (such as cell 500 of FIG. 5A) may be assigned to differentDM-RS base sequences. In some implementations, orthogonality betweendifferent cyclic shifts of different base sequences is not guaranteed,but the number of available DM-RS sequences is increased. Accordingly,the size of the cell can be increased and more UEs can be served on thesame time-frequency resource.

PUCCH & PRACH Transmissions

For PUCCH transmissions, for example for transmitting HARQ ACK/NAKs orChannel State Information (CSI), different UE transmissions in differentcells use different base sequences to avoid collisions among UEtransmissions in the different physical cells. This can be achieved byensuring that the Cell-IDs used by neighboring cells do not overlapmodulo 30. Group hopping, a feature of the LTE standard, can also beused to randomize the interference between the PUCCH transmissions fromdifferent UEs in different physical cells.

Orthogonal cyclic shifts of the base sequences (and possibly OCCs) areused in PUCCH transmissions to allow multiple UEs to transmit on thesame time-frequency resources. In some implementations, it is possibleto reuse the cyclic shifts (and OCCs when used) in different parts ofthe cell to increase the number of UEs that transmit at the same time.RF isolation can be used by the controller to determine which UEs mayreuse the one or more base sequence cyclic shifts and orthogonal coversfor the same time-frequency resource based on transmissions receivedfrom the UEs, for example, in PRACH (Physical Random Access CHannel) orPUCCH or PUSCH transmissions.

In some implementations, the interference between a cell (e.g., anysingle cell described previously) and any nearby macro cells (e.g., amobile network providing coverage outside site 10 in FIG. 1) israndomized and kept small. In some implementations, the CU chooses basesequences for use in PUSCH or PUCCH transmissions that are differentfrom the base sequences used in nearby macro cells. Furthermore, the CUcan also implement group hopping.

In some implementations, it is also possible for two or more UEs thattransmit on the Random Access Channel (RACH) using the same preamble tobe detected by the radio network of the present disclosure. Each cellwill have 64 preambles available in every PRACH opportunity. Byindividually processing the received signals from each RU or group ofRUs, the controller may, in some cases, reliably detect multiple PRACHtransmissions that use the same preamble and that are free ofsignificant interference among them. For example, referring to FIG. 5B,the controller 550 individually may process the signals from each RU orgroup of RUs (e.g., virtual cells 508 a, 580 b, 508 c) to detectmultiple PRACH transmissions 552, 554, 556 that use the same preamble.

PRACH Reuse

In a single-cell system, the PRACH opportunity is shared among all UE'sin the cell. When two UEs in a single-cell system send the same preamblein the same PRACH opportunity, it is possible for the RUs to detect one,both or none of the transmissions, depending on the relative signalstrength of the received signals in different RUs. The transmission ofthe same preamble appears to a PRACH detector as multipath. The CU candecide that the same PRACH preamble received by different RUs belong totwo different UEs based on PRACH received signal quality metrics fromall RUs.

When the CU determines that it has received the same preamble from twoUEs via two different sets of RUs and that it can resolve the contention(described below) to allow both UEs to connect, the CU sends twoseparate RA (Random Access)-Response messages to the same RA-RNTI(Random Access Radio Network Temporary Identifier) via different RUs.PDCCH and/or PDSCH reuse may be used in sending these two messages,which carry two different temporary RNTI values and the associateduplink grants may allocate non-overlapping RBs. In transmitting theRA-Response in the above manner, both UEs look for a downlinktransmission to RA-RNTI. In some implementations, the PDCCHtransmissions associated with the RA-Response are sent at an aggregationlevel of eight, e.g., to provide reliability and different RA-ResponseDL-SCH messages are sent on the same RBs. This transmission strategy canallow correct reception by the UEs to be achieved with implementation ofreuse. Provided that the UEs can correctly receive their respectiveRA-Response messages, the UEs can respond with different Message 3transmissions and proceed to set up separate RRC connections. In someimplementations, the CU may send a single RA-Response message andproceed with a standard contention resolution procedure where only oneUE is able to connect to the controller and transition to theRRC-connected state.

In some implementations, at least some part of PRACH reuse is carriedout by the RUs. For example, the preambles can be detected the RUs. Insome implementations, the detections of the preambles are carried outwithout overloading the RUs.

PUCCH Processing in the RUs & PUCCH Reuse

As explained elsewhere herein, in some implementations, a communicationcell includes a controller or controller unit (CU) and multiple RUs thatare in communication with the CU. In some implementations, at least somebaseband processing is performed in the RUs. This baseband processingmay be distributed (e.g., spread) across the RUs, as described below.The baseband processing may include one or more types of processinginvolving the LTE PUCCH (Physical Uplink Control Channel). A descriptionof the PUCCH for LTE Formats 1 and 2 is provided below, followed by adescription of the processing that may be performed by the RU and thedistribution of that processing across RUs.

Generally, Format 1 carries DL (DownLink) Scheduling Requests (SRs) andDL HARQ (Hybrid Automatic Repeat Request) ACK/NAK (Acknowledged/NotAcknowledged) signals. Generally, Format 2 is used for CSI (ChannelState Information).

PUCCH transmissions by a UE occur in subframes known to the CU. Thisalso applies to SR, although the absence of an SR is indicated bysending the “0” symbol, or by not transmitting at all. PUCCH is sentusing time-frequency resources at the edges of the band. These resourcesare allocated in chunks of 1 Physical Resource Block (PRB) at a timeover 2 slots, where the slots lie in diagonally opposite ends of thetime-frequency grid.

Format 2 occupies the NPUCCH(2) outermost PUCCH regions of the PRBs,where NPUCCH(2) is a semi-static parameter broadcast in an SIB (SystemInformation Block). All RRC-Connected (Radio ResourceControlled-Connected) UEs are assigned a Format 2 resource for periodicreporting of CSI. In some implementations, up to twelve UEs cansimultaneously (in the same subframe) send CSI in the same PUCCH regionusing a unique set of phase rotations (or a unique cyclic shift in timedomain) of a single cell-specific sequence.

In some cases, CSI user capacity can be increased in the communicationcell by using a longer CSI period and assigning UEs to non-overlappingtime offsets, or by increasing the number of PUCCH regions assigned toCSI. CSI period and offset are UE-specific parameters that are typicallyassigned at the time of connection set-up. For example, using a CSIperiod of 20 subframes, 240 (20×12), RRC-Connected UEs can send periodicCSI in one PUCCH region. In another example, in the shortest allowed CSIperiod of 2, only up to 24 (2×12) UEs can send periodic CSI on 1 PUCCHregion. Increasing the CSI period is generally acceptable in indoorsystems because of low mobility; however in the example processesdescribed herein, a longer CSI period may also increase the frequencysynchronization requirement between RUs (in simulcast and CoMP(Coordinated MultiPoint Transmission and Reception) joint transmissionscenarios). As the network communication cell load changes, the CU mayadjust the parameter NPUCCH(2) to reduce PUCCH overhead. When the CUdetermines that Format 2 resources utilize much less than NPUCCH(2)PUCCH regions over a period of time, the CU can change the PUCCHconfiguration in the System Information Block(s) (SIB) broadcast by theCU.

Format 1 occupies one or more PUCCH regions immediately following thePUCCH regions assigned to Format 2. The number of RBs (Resource Blocks)assigned to Format 1 can vary dynamically. However, when PUSCHfrequency-hopping is used, there is a parameter (PUSCH Hopping Offset)which will limit the number of RBs used for PUCCH. In some cases, up toa total number of 1024 unique Format 1 resources can be assigned, and12, 18 or 36 Format 1 resources can share the same PUCCH region of 1 RB.When the channel seen by the UE is flat across the 12 subcarriers of aPRB (Physical Resource Block), 36 orthogonal Format 1 resources can besupported, although this number may drop to 18 or 12 when the channelvaries significantly within a PRB. The number of orthogonal resourcesthat can be assigned to a PUCCH region is a cell-specific parameterbroadcast as a SIB (Common PUCCH Configuration).

Format 1 cyclic shift resources are used for SR and HARQ ACK/NAK. SRresources are reserved and like Format 2 resources, they generally areassigned to every RRC-Connected UE. When an RRC-Connected UE does nothave an assigned SR resource, it may use the PRACH (Physical RandomAccess Channel) to request uplink resources. This parameter is anothercell-specific parameter advertised in a SIB that specifies the number ofFormat 1 resources that are reserved for SR. SR configuration alsoincludes a period and an offset, which can be used to increase the SRcapacity without increasing the number of PUCCH regions reserved for SR,although increasing the SR period also increases the average access timeon the UL (UpLink).

Described below are example processes for performing PUCCH (PhysicalUplink Control Channel) decoding in RUs. In the example systemsdescribed herein, the CU localizes the positions of RRC-connected UEs toa relatively small number of RUs. Localization determines which “local”RUs are to serve which UEs in the communication cell. Localizationenables the decoding of PUCCH in the RUs. According to the exampleprocesses described herein, after the CU determines the “serving” RUsassociated with each RRC-connected UE, the CU can send information(called “side information) for use in PUCCH decoding to the RUs prior tothe beginning of each subframe of communication. The information mayinclude the basic PUCCH configuration parameters such as NPUCCH(1) andNPUCCH(2) and the Format 1 and Format 2 cyclic shift resource indicesfor each resource to be decoded. The decoded information, including theresource index, is sent back to the CU. Since Format 1 does not use anychannel coding, the RU may send a soft-decision metric to the CU, andallow the CU perform inter-RU combining to increase reliability. In someimplementations, the RU performs the combining across its two local Rxantennas.

Format 2 uses a (20, A) block code in this example implementation, whereA is the number of bits in the CSI (which varies based on CSI Format)and 20 represents the number of coded bits. In this example, the RU maydemodulate up to the coded bits and send the coded bits to the CU alongwith a single quality metric. The RU does not need to know the number ofbits in the CSI. In an example, assuming a 6-bit quality metric, at most12×(20+6)=312 bits need to be sent per PUCCH region, or 312 kbps.Alternatively, each RU may decode the CSI completely, and send thedecoded CSI to the CU along with a quality metric, which the CU may useto select one RU's data.

As explained above, SR and CSI are the main contributors to capacityconstraint on PUCCH. One process for addressing this issue includesreusing cyclic-shift resources in different parts of the communicationcell.

In this regard, when two UEs in the same cell use the same Format 1resource to transmit SR, there is some possibility of a collision. Forexample, suppose two UEs (UE #1 and #2) share the same SR resource, andUE #1 transmits SR in some subframe and UE #2 does not. If the RUs thatserve UE #2 can receive UE #1's transmission even at a very low signallevel, the CU may declare a “received SR” for UE #2. This may cause theCU to unnecessarily allocate PUSCH resources for UE #2. To avoid thisproblem, the CU can compare the “quality” of the PUCCH received signalfor UE #2 SR by comparing it with the signal level previously reportedfor UE #2 by its serving RUs.

Since HARQ resources grow with the number of users per TTI, the HARQresources will grow linearly with the number of users co-scheduled onthe DL. Since HARQ Format 1 resources are determined based on the CCEused for PDCCH, any reuse of PDCCH will automatically result in acorresponding reuse of the HARQ resource inside the same cell. Forexample, when two UE's (UE #1 and #2) are served on the DL using thesame PDCCH CCE, they will automatically share the same HARQ resource totransmit ACK/NAK. In one scenario, UE #1 may transmit an ACK and UE #2transmit a NAK in some subframe. Since both UEs will be transmitting thesame Format 1 DM-RS in OFDM symbols 2, 3 and 4 of each slot, the PUCCHdecoder in a given RU will estimate the channel to be the sum of the twochannels (from the two UEs), e.g., H1(k)+H2(k). But in the other OFDMsymbols that carry the ACK/NAK bit, when the two UEs are transmittingdifferent HARQ bits (X1(k)=−X2(k)), the receiver will see the differencechannel H1(k)−H2(k). This renders the equalizer suboptimum and resultsin a bias that reduces the SINR.

Enhanced Detector for PUCCH

As explained above, “reuse” includes, but is not limited to, two devicesin a single cell utilizing the same resource (e.g., frequency) forcommunication within that cell. The “reusing” device may be the remoteunits (RUs), the user equipment (UEs) (e.g., a mobile device), or anyother appropriate device. Reuse may occur on the downlink (DL) or on theuplink (UL), as described herein. On the UL, e.g., for reuse of thePhysical Uplink Control Channel (PUCCH), two or more UEs may communicateon the same resource. In some implementations, that resource may befrequency; however, other resource(s) may be used. Because different UEstransmit on the same frequency, it may be necessary to separate signalsfrom different UEs at a receiver (e.g., at a Base Station (BS)). In someimplementations, detection of different signals on a same resource isperformed based on the Radio Frequency (RF) distance between thosetransmissions. Described below are examples of a detector used to detectsignals from different UEs on the same resource, and processes that maybe implemented by the detector.

In some implementations, RUs may include a single antenna forcommunication with various UEs, whereas in other implementations, RUsmay include two or more antennas for communication to various UEs. Inthis example, there are single-antenna RUs (RU1 and RU2), incommunication with UEs (UE1 and UE2). In this example, PUCCH signalstransmitted by UE1 and UE2 are denoted by s1 and s2 and can have values(−1, +1, 0) corresponding to (ACK, NACK, DTX) signals with givenprobabilities (Pr), where ACK refers to an acknowledged signal, NACKrefers to a not acknowledged signal, and DTX refers to DiscontinuousTransmission. In this example, it is assumed that Pr(ACK)=0.81,Pr(NACK)=0.09, and Pr(DTX)=0.1. Furthermore, in this example, UE1 andUE2 are either in the same cell, thus sharing the same PUCCH resources,cyclic shifts, and orthogonal cover codes, and transmitting the samereference signals; or UE1 and UE2 are in different cells, and thus aredistinguished by different cyclic shifts, and orthogonal cover codes,and transmitting different reference signals.

FIG. 27 shows an example implementation showing communication betweenRU1, RU2, UE1 and UE2. The example processes described herein are usablewith different types of receive (RX) antennas, including (1)uncorrelated RX antennas, and (2) correlated RX antennas.

Single-User Detector without Reuse

A single-user PUCCH detector declares DTX if the detected received powerP<T_A times N, where T_A is a configurable threshold value, and N is thesum of the thermal noise and other-cell interference measured at the RP.In the absence of reuse, the probability of False Alarm Pr(FA) dependsonly on the threshold T_A, and is independent of the signal or noiselevels. In some implementations, T_A is chosen such that Pr(FA)<0.01,where Pr(FA) stands for the probability of false alarm. Generally, apriori knowledge about the SNR does not always help in choosing thethreshold T_A. For the chosen T_A, the probability of missed detection,Pr(Miss), increases with a decreasing SNR. In some implementations,there is a minimum SNR, SNR_(min), Pr(Miss) remains below a target of1%. The optimum value of T_A for single-user detection with no reuse isalso referred to as T_A_NR.

Single-User Detector with Reuse

When there is reuse, assuming that UE2 is transmitting an ACK or a NAK,Pr(FA) for UE1 can be written as:Pr(FA)=1−Pr(Miss-Int)<1%; or equivalently: Pr(Miss-Int)=1−Pr(FA)>99%.Here Pr(Miss-Int) represents the probability that the interfering signalis missed. Since a receiver of a detector cannot necessarily distinguishbetween the two UEs that broadcast the same reference signal, for thedetector to not misfire on DTX of UE1, it has to not detect UE2. For agiven threshold T_A, Pr(Miss-Int) depends on INR=SNR/SIR, which is theinterference-to-noise power ratio at the receiver for UE1. Usingavailable knowledge about INR from localization measurements, athreshold value T_A can be determined to satisfy the P_FA conditionabove (e.g., Pr(Miss-Int)>1−P_Pr(FA)>99%). For example, assuming SIR=20dB and 30>SNR>−5 dB, it follows that 10>INR>−25 dB. The value of thethreshold T_A thus increases with INR. At high values of INR, thethreshold T_A can be increased significantly to keep Pr(FA)<1%.

It also may be desirable to satisfy the condition Pr(Miss)<1%. Once T_Ais chosen according to INR to meet the Pr(FA) condition, there is aminimum SNR, SNR_(min), above which Pr(Miss) condition can be satisfied.The value of SNR_(min) can be determined via simulations. To satisfyboth conditions Pr(FA)<1% and Pr(Miss)<1% at SNR_(min)=−5 dB, SIR iscontrolled to be within a desirable range.

When both UEs are in DTX, Pr(FA) for the single-user detector willdecrease, because of the use of a higher threshold T_A. When only UE2 isin DTX, however, the probability of miss will increase, but generallywould remain below the 1% target, provided SIR is large enough.

Multi-User Detection with Reuse

In some implementations, a joint detector can operate with twothresholds, T_A and T_B, where T_B>T_A, as follows:

Initially, in a first operation, if (P1+P2)<T_A (N1+N2), both UEs aredeclared to be in DTX. Otherwise, in a second step, based on thedetermination that at least one UE has signal (ACK/NAK), P1 and P2 arecompared:

-   -   If P1>P2, UE1 is declared to have ACK/NAK; what is left to be        determined is whether UE2 is DTX or not;    -   If P2>P1, UE2 is declared to have ACK/NAK; what is left to be        determined is whether UE1 is DTX or not.        Sometimes an error will not occur in this operation, if the        SIR>0 dB. Next, if in a second operation P1>P2, and P2>T_B times        N2, no DTX is declared. Otherwise UE2 is declared to be in DTX.        If in the second step, P1<P2, and P1>T_B times N1, no DTX is        declared. Otherwise UE1 is declared to be in DTX.        Localization

When receiving the UE's uplink signal at one or more RUs, or in reusescheduling, it may be desirable to determine the radio location of a UEwith respect to the RUs. Localization includes techniques used to makethis determination. In some implementations, localization can be basedprimarily on detection of the UE's uplink signal. In someimplementations, UE measurement reports of the downlink signal can alsobe used to aid the process. Localization processes can be used in one ormore of the following features associated with the cell(s).

Pruning for PUSCH: Generally, at any time point, a UE is in the radiovicinity of only a subset of RUs that belong to a cell. Hence, the CUdecides on this subset of RUs from which to receive the UE's uplinktransmissions. The CU then combines the received signals from these RUs.Generally, the larger the pathloss from a UE to an RU, the weaker thereceived signal, which may diminish returns from soft-combining thereceived signal from such RUs. The process of selecting a suitablesubset of the RUs by a CU is called pruning. In some implementations, ineach TTI, the CU provides each RU with the list of RB's to receive fromwith the RNTI to PRB assignments, thus providing the RU the informationit needs to perform pruning. In some example applications, which arealso described earlier, the RU may demodulate, and possibly decode, thereceived PUSCH signal. In this case, pruning determines the set of RUsthat demodulate and possibly decode the received PUSCH signals on eachRB. In this example, only RUs in the pruning set forward PUSCH data tothe controller for further processing.

Location-aware PUCCH: In some implementations RUs may demodulate anddecode the received PUCCH transmissions from the UEs. A similar functionto pruning can be performed for the PUCCH, where the number of RUs thatprocess a UE's PUCCH can be reduced using localization information.

Uplink load balancing across RUs: Each RU can handle a certain number ofuplink PUCCH UCI such as SRs, HARQ ACK/NACKs, etc. per TTI based on theRU's processing limitations. A localization module can provideinformation that helps with load balancing across RUs in an equitablemanner. Examples of load balancing tasks may include, but are notlimited to: mapping of UEs to RUs, or limiting max number of scheduledUEs per RU in such a way as to balance the HARQ load per RU (if HARQinformation can be decoded successfully from multiple RUs); andassignment of SR and CSI resources to a UE so that the per-TTI load oneach RU is balanced across the cell.

Downlink and uplink reuse: As described previously, data may betransmitted to multiple UEs or received from multiple UEs in the samePRBs at the same time. A subset of RUs in the cell can be assigned toserve each UE.

Localization Metrics

The received energy at the RU, the SINR or path gain can be used as themetric for localization. In some implementations, the metrics forlocalization are determined using the received PRACH and SRS signalsfrom a UE at each RU. In addition, the PUCCH and PUSCH DM-RS signals canalso be used for localization.

PRACH-Based Localization

When a UE attempts a random access, it transmits a random accesspreamble using resources known as PRACH. These transmissions can beeither contention-based or contention-free. The former occurs when theUE has not yet established a connection with the eNodeB, while thelatter occurs when the eNodeB allocates a specific PRACH resource to theUE, for example during handover.

When a UE transmits a PRACH sequence with sufficient power, the eNodeBdetects the sequence, and responds with a temporary RNTI (TC-RNTI, RadioNetwork Temporary Identifier) along with a resource allocation for theUE to transmit further information regarding this access attempt. Atthis stage, the eNodeB does not yet know the identity of the UE.Contention resolution takes place when the UE transmits its identity inits first message over the UL-SCH (uplink shared channel) using theallocated resources.

Assume that UE₀ transmitted a PRACH sequence p_(l), and that a set of MRUs [RU_(m1), RU_(m2), . . . RU_(mM)] are able to detect the sequencetransmitted by the UE, although all RUs in the cell will be attemptingto detect PRACH sequences during the allotted PRACH opportunities. Thedetected sequence can be used to determine a metric that indicates thestrength of the preamble signal as it is received by the RU. Such ametric can serve as a relative measure for localization purposes. Theuplink pruning can be valid for the subsequent uplink transmissions bythe UE, until further localization information is available from the UE.

PRACH Contention

In some implementations, contention can happen on a particular PRACHresource, e.g., more than one UE transmits the same PRACH sequence inthe same opportunity. In this situation, the localization process candetermine the pruning set from the superset of RUs receiving from themore than one UE. Subsequently, at the time of contention resolution,the localization process can determine whether a PRACH contention hadtaken place and thus remove the records stored for the UE for whichcontention resolution had failed.

SRS Based Localization

SRS (Sounding Reference Signals) are transmitted by UEs at specifictime-frequency resources as configured by the controller. The SRS can beused for keeping track of the link quality from a UE to scheduleresources for PUSCH efficiently when needed. A UE transmits SRS when itis active or in its DRX (discontinuous reception)-wake-up intervals. TheSRS can also be used for localization purposes. For example, either allthe RUs or a subset of the RUs can be configured to receive SRS from aUE in a given instance.

The transmit power for SRS is:TxP=P ₀ +αPL+f(Δ_(TPC))+10 log₁₀ M,where P₀ is the open loop power, PL is the estimated pathloss and α isthe fractional pathloss component, f(Δ_(TPC)) provides the accumulationof closed loop power control commands, and M is the number of RBs overwhich the SRS is transmitted. The power control commands (TPCs) used forthe SRS are the same as those used for the PUSCH.

Localization based on SRS for the purpose of uplink pruning can also bebased on the relative energy received by each RU from the SRS of a UE.For the purposes of relative strength measurements and absolutemeasurement of path loss or channel gain, the localization module mayalso obtain information about the transmit power of the SRStransmission. In addition, since each UE transmits its SRS periodically,the SRS measurements can be accumulated at the localization module toprovide a smoothed estimate of the received signal strength from a UE.The SRS might be in frequency hopping mode, in which case, thelocalization module can perform both time and frequency domainaveraging.

The SRS signal from a UE can be periodic or aperiodic. In an exampleimplementation of the periodic case, the SRS can be of periodicitybetween 2 and 320 ms. Generally, the shorter the periodicity, the morefrequently the RUs will receive the SRS measurements and hence betterthe accuracy in tracking and localizing the UEs. If the configuredperiodicity for SRS transmission is 320 ms, the UE sends a periodic SRSevery 320 ms. The SRS opportunities may also be staggered in time asevenly as possible among the connected mode UEs so that the RU and theCU SRS processing load is even. The same periodicity may be configuredto the connected mode UE in the system but the SRS trigger opportunitiesin time may be as evenly spaced as possible. Taking the example of a 320ms periodicity, there are 320 different SRS opportunities spaced equallyin time within those 320 ms.

In some implementations, it may be beneficial to position and locate theUE as early as possible. Using a large SRS periodicity may mean lessfrequent SRS receptions and the RUs and CU and hence a lesser rate oftracking the UEs. At the very start of a UE's connection with the CU,the UE's location is not known with a lot of accuracy and hence coarse.The more SRS measurements the CU receives from a given UE, progressivelythe more accurate that UE's location is ascertained.

Using the example of a 320 ms periodicity, it was noted earlier thatthere are 320 different SRS opportunities. To enable a UE to trigger aSRS quickly or on demand, 20 equally spaced SRS indices are reserved.These 20 indices are spaced 16 ms apart. For convenience, these 20 SRSindices are termed Group-A SRS indices. The remaining 300 indices arefor convenience termed Group-B SRS indices. A SRS index and a SRSopportunity are synonymous.

In some implementations, at the very start of a UE's connection, onefree SRS opportunity from the Group-A SRS indices is assigned to the UE.In LTE, this configuration may happen via the RRC CONNECTION SETUPmessage. This Group-A SRS resource is configured to the UE and informedto be a single transmission opportunity. In other words, Group-A SRSopportunities are non-repeating. This enables the UE to transmit the SRSwithin approximately the subsequent 16 ms. The location of the UE isknown at the CU with greater accuracy than before and helps withtransmission pruning on the uplink.

Up to eight such newly arriving UE into the system can be configured totransmit on the same Group-A SRS index by configuring one of 8 differentphase rotations to each UE. The phase rotation is also referred to ascyclic shifts in the LTE standards. To enable further tracking of theUEs that transmitted a non-repeating Group-A SRS, a periodic Group-B SRSindex is configured immediately after the Group-A SRS was received. InLTE, this is done by signaling the Group-B SRS index to the UE via a RRCCONNECTION RECONFIGURATION message.

Where it is deemed that a UE needs to be tracked more frequently thanthe granularity provided by Group-B SRS, the CU can reconfigure the UEtemporarily with a Group-A SRS index for a single shot aperiodictransmission before moving the UE back to its Group-B SRS configuration.

It is also possible to remember localization measurements betweensuccessive RRC connections of the same UE using the so-called S-TMSIidentifier. S-TMSI is a UE identity (known to the Evolved Packet Core orEPC), which is unique within certain “duration”. A UE's S-TMSI is(re-)assigned by the MME and becomes known to the CU when the UE sendsan RRC Connection Request to enter the RRC-connected state. A basebandcontroller can keep a database of localization information (e.g.,signature vectors, or information related to path gains to differentRUs) indexed by S-TMSI for all recently RRC-connected UEs. At connectionrelease, the database entry for the corresponding S-TMSI is updated.When a UE first connects, the CU retrieves the stored localizationinformation for the corresponding S-TMSI and checks whether thelocalization information obtained from PRACH is consistent with thestored localization information. If it is, the CU proceeds to use thestored localization information to initialize the localizationprocedure. Otherwise, it initializes the localization procedure usingthe PRACH based measurement.

In addition to the above, the UEs are also requested to send periodicpower head room (PHR) reports to the CU. The power head room reportsallow the CU to estimate the pathloss. Controller may use pathlossmeasurements in the localization algorithm.

Localization and Pruning Processes

In an example, assume that the localization metric is maintained basedonly on the SRS, and PUSCH/PUCCH-based energy estimates are not used.

FIG. 24 provides a block diagram view of an example localization processfor pruning. The elements for reuse implementation are also identified.For each UE, an RU can belong in one of two sets from the pruning pointof view: ActiveAndCandidatePruningRPSet, and OtherRPSet. The RUs in theUE's OtherRPSet can be tracked at a slower rate than those in theActiveAndCandidatePruningRPSet. This can reduce the SRS measurement loadon the system, thus allowing for a tradeoff that enables the SRSperiodicity to be decreased, e.g., measure at a faster rate, ifnecessary. This can be disabled by setting the appropriate parameter(s)such that all RUs measure at the same rate. The details of theparameters are described further below. Moreover, in someimplementations, a localization database keeps entries only for thoseRUs that are in the ActiveAndCandidatePruningRPSet, and does notmaintain entries for RUs in the OtherRPSet. Using the OtherRPSet toprune SRS reception by RUs can be an optional implementation. Forexample, the SRS may be processed from all RUs at each opportunity,including the OtherRPSet, but only the ActiveAndCandidatePruningRPSetRUs' measurements will be maintained as a moving average. The OtherRPSetmeasurements can be discarded unless they meet the criteria to beincluded into the ActiveAndCandidatePruningRPSet.

Within the ActiveAndCandidatePruningRPSet, an RU can be in an Active ora Candidate state with respect to the PUCCH or PUSCH, creating, e.g.,four possible combinations of sub-states that can be captured with twobits. One member of the ActiveAndCandidatePruningRPSet can be designatedas the Primary RP. The other members can be assigned toActivePucchPruningRPSet, CandidatePucchPruningRPSet,ActivePuschPruningRPSet and CandidatePuschRPPruningRPSet as applicable.

In a localization process, there may be two phases—initialization andmaintenance, and a different set of parameters is used in each of thesephases. This differentiation can allow for a greater tolerance in themeasurements in the initial period when there has not yet beensufficient averaging over the unknown fast fading. FIG. 25 is a blockdiagram showing example maintenance of a pruning set. The figure showsinteractions between various modules of the system and also provides anoverview of the order of events in the execution of localization andpruning. For example, the UE transmits its PRACH or SRS as applicable.The relevant RUs measure the transmission, and provide the measurementto the localization module via the RU-CU interface. The localizationmodule can execute the pruning process, and create/update thecorresponding table in the localization database. When updates are madeto the any of the sets mentioned above, the RRM (radio resourcemanagement) and MAC/Scheduler modules are notified, so that they canretrieve the revised pruning table from the database.

Location-Aware PUCCH and Overload Control

Described below are examples of PUCCH (Physical Uplink Control Channel)processing performed in a remote unit (RU) taking advantage oflocalization. The example processes may be implemented to control theload across RU's, that is both to make sure that each RU stays withinits processing limits, and that the load is balanced across RU's tomaximize overall capacity. In this regard, as explained elsewhereherein, in some implementations, a communication cell includes acontroller or control unit (CU) and multiple RUs that are incommunication with the CU. In some implementations, at least somebaseband processing is performed in the RUs. This baseband processingmay be distributed across the RUs to reduce the chances that any singleRU will become overloaded and thereby act as a bottleneck in the cell.This distribution is referred to herein as “load balancing”. Examples ofuplink (UL) data processing (e.g., baseband processing) that may beperformed in the RUs is described below, along with exampleload-balancing processes.

In an example implementation, at the beginning of each subframe duringcommunication between a CU and RUs, the CU sends information to all RUsidentifying the uplink, e.g., PUCCH, resources that each RU is toprocess. In some implementations, this information is individualized(e.g., different) for each RU, but is sent in a single multicastmessage. In this example, the CU knows, beforehand, the availableprocessing resources in each RU and makes overload control decisions forthe various RUs prior to sending the information.

An example implementation of a cell includes J (J>1) RUs. Each RU iscapable of processing K (K>1) PUCCH resources per TTI (Transmission TimeInterval), where the PUCCH resources may include, e.g., SR (SchedulingRequests) or CSI (Channel State Information) or Hybrid Automatic RepeatRequest Acknowledgements (HARQ ACK/NACK). In this example, J×K SR or CSIresources are processed per TTI across all RUs. In this example, if theSR or CSI periods are designated as P, the maximum number of RRC (RadioResource Controlled) connected UEs (N_connected) that can be supportedby all RUs in the communication cell, for a given PUCCH format, is asfollows:N_Connected=J×K×P.

-   -   For J=20, K=4, and P=20, N_Connected=1600.        In the above example, we assumed that the 1600 connected UEs are        relatively evenly split among the 20 RUs so that there are 80        connected UEs per RU.

In some implementations, UEs are randomly distributed across acommunication cell and, as a result, the number of connected UEs in thecoverage area of each RU will also vary randomly. For a givenprobability distribution for the UEs, it is possible to determine theprobability of exceeding the nominal number K of PUCCH processingresources in an RU. For example, with a uniform probability distributionand 40 connected UEs per RU on average in a cell (50% of the exampleabove), in a given RU, there is about a 4% chance of needing to processmore than 4 (either SR or CSI) resources, assuming a period of P=20, andusers evenly distributed among the SR and CSI phases across the entirecell.

In some cases, it may be possible to improve upon the performance shownin the above example by taking advantage of UE localization in assigningSR/CSI phases. In this regard, in some implementations, the CU canmanage the PUCCH processing load (e.g., the amount of PUCCH informationto process by the various RUs) dynamically to reduce the chances ofoverloading one or more of the RUs. Example processes for managing thePUCCH load dynamically are described herein. In some implementations, asingle process may be used to manage the PUCCH processing load. In someimplementations, two or more of the following processes may be used tomanage the PUCCH processing load. In some implementations, the PUCCHprocessing load may be managed by the Localization and Pruning Module ofFIG. 24

An example process for managing the PUCCH load dynamically is referredto as “Load-Dependent PUCCH Periods”. According to this process, atRRC-Connection set-up, a CU can assign the CSI and SR periods accordingto the system (cell) load. For relatively light system loads, the CUkeeps P_(SR) and P_(CSI) relatively small to enhance performance. As thesystem load increases, the CU increases P_(SR) and P_(CSI) for new UEs.As an alternative, the CU may also decide to reconfigure both old andnew UEs with the most applicable P_(SR) and P_(CSI) based on currentloading conditions.

Another example process for managing the PUCCH load dynamically isreferred to as “Location-Dependent PUCCH Resource Assignment”. Accordingto this process, at RRC-Connection set-up, the CU will assign UEs in thesame RU coverage area (or nearby RUs) to different CSI and SR phases, sothat transmissions to/from those UEs are not processed in the same TTI.As a result, UEs that are in the coverage area of an RU may be evenlydistributed between CSI/SR phases, at least when there is no mobility.Mobility may create uneven distributions, which may be addressed byreconfiguration.

Another example process for managing the PUCCH load dynamically isreferred to as “Load-Dependent PUCCH Uplink Combining”. According tothis process, at each TTI, the CU instructs RUs to process specifiedPUCCH resources. During times when there is a relatively heavy-load loadin the cell (e.g., over a predefined amount of communication traffic),the CU may limit the processing of each PUCCH resource to one RU.Otherwise, the CU may allow a PUCCH resource to be processed by multipleRUs and the results of the processing by the multiple RUs may becombined.

Another example process for managing the PUCCH load dynamically isreferred to as “Dynamic Purging of PUCCH Resources”. According to thisprocess, when the CU determines that an RU does not have enoughprocessing resources to handle its designated PUCCH processing, the CUselectively discards processing of some CSI and/or SR resources. In someimplementations, a scheduler in the CU may implement a form of overloadround-robin scheduling so that the CSI or SR misses are evenlydistributed across all UEs. A single SR or CSI miss for a UE amounts toa temporary doubling of the corresponding period, as long as the sameresource is processed in the next period, and can be viewed as a dynamicform of overload control. While deciding to miss SR or CSI processingfor a UE due to an overload condition, in some examples, the CU maypreferentially handle the SR or CSI processing on the subsequentopportunity for the same UE. This example process may be performed onlyon those RUs, leaving other RUs in the cell unaffected.

In some implementations, the CU will accord the highest PUCCH processingpriority in the RUs to HARQ ACK/NAKs (Hybrid Automatic Repeat RequestAcknowledged/Not Acknowledged). How many HARQ PUCCH processes areassigned to an RU in a given TTI ultimately determines the number of SRand CSI resources that can be processed by that RU. For example, ifK_(tot) is the total number of PUCCH resources (HARQ, CSI and CS) an RUcan process, the CU will constrain the scheduler so as to not schedulemore than K_(tot) UEs in the same RU/TTI. If K_(HARQ) is the number ofPUCCH resources needed for HARQ in a given TTI, K_(tot)−K_(HARQ) will bethe number of SR and CSI resources that can be processed in the sameTTI/RU.

If the transmission of an HARQ ACK/NAK by a UE falls on the same TTI asan SR or a CSI, there are mechanisms within the LTE specifications toallow the UE to send both on the same resource (simultaneous CSI+ACK/NAKtransmission is a configurable option). In these cases, the CU may alertthe RU to apply the simultaneous SR+ACK/NAK or CSI+ACK/NAK detectionprocesses.

Another mechanism for managing the processing load on the RU is to havethe Pruning and Localization module fine-tune the pruning list asnecessary. For example, the pruning module first uses theActivePuschPruningRPSet and ActivePucchPruningRPSet for each UE as thebaseline in determining an RU pruning set. The pruning module also keepsaccount of the number of PUCCH format 1 and PUCCH format 2 receptions ateach RU on a per subframe basis. Based on preconfigured rules regardingRU loading, the pruning module can then perform additional pruning,e.g., by deleting additional non-primary RUs from individual UE'spruning sets to stay within each RU's loading limits. Examples ofdifferent loading limits include:

-   -   Limit 1: Maximum number of PUCCH format 1 HARQ messages per RU    -   Limit 2: Maximum number of PUCCH format 2 messages per RU    -   Limit 3: Maximum (format 1 HARQ+format 1 SR+format 2) messages        per RU

In some implementations, a limit on the number of format1 messages isalready placed by the MAC/Scheduler. If any of the loading limits iscrossed by an RU, then the list of messages to be processed at the RU isfurther pruned. This further pruning can be performed based on one ormore the following rules:

The Pruning and Overload Manager module may keep track of the RNTI's ofthe last (X) UE's, whose PUCCH messages were pruned at each RU, andattempt to select a UE that is not in that list. If there is a conflictbetween pruning out a CQI (format 2) message and an SR, the CQI messageshould be selected for pruning out. If a UE's CSI or HARQ feedback onthe uplink were to align with a PUSCH transmission, the CSI or HARQfeedback is multiplexed with the PUSCH transmission and sent to the CU.In this instance there is no PUCCH transmitted by the UE. The CU notesthis condition at the time of scheduling and notifies the RUsaccordingly.

In summary, in LTE, there are three types of control informationtransmitted on the uplink control channel, PUCCH. The first type is HARQACK/NACK (hybrid automatic repeat request), which can require 1 or 2bits. The second type is CSI (channel state information), which includesCQI, PMI, and RI. The CSI can be sent every 2 to 160 ms though smallerranges such as 2 to 20 milliseconds are likely to improve downlinkperformance. The third type is SR (scheduling request). The SR from a UEinforms a scheduler that there is data from the UE to be transmitted onthe uplink.

The PUCCH processing is distributed between the CU and its RUs. Someparts of the baseband processing associated with PUCCH can take place atthe RUs. For example, the CU can inform the RU to determine which onesof the UEs to watch for uplink transmission. Different controlinformation can be sent on PUCCH at different frequencies or ondifferent “cyclic shift” resources on the same frequency. In someimplementations, the different frequencies or cyclic shifts aredetermined at least partially based on load balancing of the RUs. Forexample, it may be desirable not to overload the RUs, and also todistribute the baseband processing load evenly over all RUs. In someimplementations, the periods and phases of the CSI and SR are assignedto evenly distribute the load over the RUs in each TTI. In someimplementations, the locations of the UEs are taken into considerationto further even out the burden on the RUs. In some implementations, atthe beginning of the PUCCH transmission, the periods for differentsignals are set to be relatively low, e.g., to provide good precision inthe transmission.

As described previously, in some implementations, at least part of PRACHprocessing takes place at the RUs, instead of the CU. In addition, SRS(sounding reference signal), can also be at least partially processed atthe RUs. The overall load control and load balancing at the RUs are alsoconsidered for processing the PRACH and the SRS.

Dynamic Coverage and Capacity Adjustment

Referring again to FIGS. 2A and 2B, the RF coverage and capacityprovided in the radio network are decoupled. The RUs 66 a-66 e, 90 a, 90b, 92 a, 92 b provide the coverage and the baseband modems 62, 82, 84,or the CUs 60, 80 provide the capacity. In some implementations, someRUs in a radio network are deployed more densely and with moretransmitter power than other RUs in order to overcome possibleinterference from nearby eNodeBs, for example, macro cells. In someradio networks of this disclosure, RUs are deployed very closely to eachother, with overlapping coverage, because they can belong to the samecell and therefore do not cause any inter-cell interference. Such verydense deployments are sometimes not possible with traditional basestations. The number of baseband modems (and cells) needed for a sitedepends on the number of users, the amount of data usage per user, andthe distribution of users across the site as a function of time, etc. Ingeneral, a minimum number of baseband modems (and cells) is used to keepthe cost low and to avoid unnecessary cell boundaries. When the demandfor coverage and/or capacity changes, the radio network of thisdisclosure can dynamically adjust its coverage and capacity.

Dynamic Capacity Reallocation

In some implementations, when multiple RUs share the same cell/basebandmodem, the capacity of the baseband modem is shared by all the UEs thatfall within the coverage area of all the RUs that are assigned to thebaseband modem. In an area of relatively high data usage, the RUs thatform the cell may cover a smaller area than RUs in another cell thatcovers an area of relatively low data usage. For example, at a siteusing 4 modems (and 4 cells) and 24 RUs, the 4 cells can have 2, 4, 8and 10 RUs, respectively, providing different cell sizes that match thecoverage and capacity demand. The assignment of RUs to the cells can bedynamically changed based on changes in capacity demand. The changes canbe made manually, e.g., by having a local person modify the RU tocontroller mapping, semi-automatically, e.g., based on Time-of-Day(ToD), or automatically, e.g., by the controller based on detecting achange in traffic distribution. The changes can reallocate the capacityat the site, without any substantial changes to the deployed equipment.

As an example, referring to FIGS. 6A and 6B, a radio network 602including three modems 604 a, 604 b, 604 c controlling three respectivecells 608 a, 608 b, 608 c through an off-the-shelf Ethernet network 606is deployed at a site 600. The site 600 can be a commercial buildingthat includes shopping areas and office space, which have differentcapacity demands (as schematically shown by different numbers of usersin the figures) at different ToD. The cells may each include differentnumbers of RUs (not shown) to cover different-sized areas, whileproviding substantially the same traffic capacity. The shapes of thecovered areas by the different cells can also be different.

Referring particularly to FIG. 6A, at a given time (time 1, e.g., workhours on a weekday), most users of the site 600 are concentrated insmall areas 610, 612 (e.g., office spaces), while the user density isrelatively low in the larger area 614 (e.g., the shopping areas). Tomeet the different capacity demands in the different areas of the site600, the cells 608 a, 608 b having a relatively small number of RUs areformed to cover the areas 610, 612, and the cell 608 c having arelatively large number of RUs is formed to cover the area 614. Eachcell 608 a, 608 b, 608 c has substantially the same capacity.

The capacity demands at the site 600 may dynamically change. Referringto FIG. 6B, at another given time (time 2, e.g., lunch hours on aweekday), there is a high density of users in areas 618, 620 (e.g.,restaurant areas in the shopping area 614 of FIG. 6A) and there arerelatively few users are in the area 616 (e.g., office areas 610, 612and store areas in the shopping area 614 of FIG. 6A). In response, oneor more RUs at the site 600 are reassigned to different modems,manually, semi-automatically, or automatically, to form new cells 622 a,622 b, 622 c that cover the respective areas 616, 620, 618. The cell 622a contains a relatively large number of RUs. The cells 622 b, 622 ccontain a relatively small number of RUs. Each cell 622 a, 622 b, 622 chas substantially the same capacity. Dynamic capacity reallocation isimplemented over the Ethernet network.

Total Capacity Increase

In some implementations, instead of or in addition to redistribution ofcapacity demands on a site (e.g., the site 600 of FIGS. 6A and 6B), thesite also experiences an increase in the demand for total capacity. Forexample, the number of mobile subscribers increases, and/or the amountof data demand per subscriber increases. In these implementations,additional modem(s) (and accordingly additional cell(s)) can beintroduced. For example, an existing unused modem in a CU of the radionetwork can be enabled and some of the RUs already deployed at the sitecan be reassigned to the new modem. This is a form of real cellsplitting, which can be implemented in a convenient manner, e.g., as asoftware upgrade, and, in some implementations, does not require anyhardware changes to the installed RUs. Alternatively or in addition, oneor more new modems can be added in a CU and/or one or more new CUs canbe added to the radio network at the site. In some implementations, thetotal capacity of the site may be increased without affecting thepreviously deployed modems, cells, and RUs. In some implementations, theaddition of more modems or CU hardware is significantly less expensive,both in terms of equipment and installation cost, as compared to addingmany new access points across the site. The physical cell splittingmethod described above is implemented using the Ethernet network.

CU Stacking

In some implementations a CU controls 64 RUs. It may be desirable toserve larger spaces or deliver higher capacity per RU while preservingthe coordination and/or no cell border properties of the system. FIG. 26shows another example of a cell which expands the coverage by stackingmultiple CUs (three shown in the example) within one single cell, whereeach CU uses a single baseband modem. The CUs (or baseband modems) arecoordinated using a coordination function (CF) and are connected amongthemselves and with the CF through an Ethernet link, e.g., a 10GEthernet.

Each CU is a physical controller that runs a complete controllerapplication, except for certain inter-controller coordination functionscan be offloaded to off-the-shelf server(s). Within the cell thatcontains the multiple CUs, there is no fixed association between CUs andRUs. Each CU application can handle a subset of the connected UEs in thecell. When an RU receives a PRACH from a UE, the RU assigns the UE toone of the available CU applications. When an MME (Mobility ManagementEntity) forwards a page to the backhaul CU, the backhaul CU assigns thepage to a CU.

Certain inter-CU dependent functions are off-loaded to an off-the-shelfserver, which can have external storage, running the coordinationfunction (CF). Virtualization may be used on this server to allow otherservice applications to run on the same hardware, and for furtherscalability. In some implementations, one single CF is implemented forall CUs of a cell, although multiple CFs can also be used.

In some implementations, the server running the CF is an Off-the-Shelf(OTS) Server. The OTS can provide flexibility in terms of processingpower, good scalability, and has no wasted physical resources.Additionally, the OTS can be used to run other applications, includinglocation-based services, local break-out and other services, etc.

Within each cell, some CUs are selected to perform common functions forall RUs. For all CUs, the associated CF is the master. One selected CUis a Timing Master CU that acts as the 1588 master for all RUs. Anotherselected CU is a System Information CU. This CU is responsible forgenerating the system information and cell-specific reference signals,e.g., CS-RS and CSI-RS, for the entire cell. The System Information CUalso schedules SI, CS-RS and CSI-RS and the scheduling can beexclusively handled by this CU. There is also a Backhaul CU that isresponsible for maintaining a single IPSec (Internet Protocol Security)and S1 tunnel to the Evolved Packet Core (EPC). Backhaul CU acts as aneNodeB terminating the S1 tunnel towards the EPC. In someimplementations, IPSec packets are tunneled through a single BackhaulCU. The Backhaul CU can also be responsible for CU selection uponreceiving a Page from MME.

The CF and the CUs together perform inter-CU coordination functions. Oneof the coordination functions is localization. In some implementations,the CF maintains localization information for all connected UEs. Eachtime a CU updates a UE's uplink signature vector, the CU forwards thenew signature vector along with a UE identifier to the CF.

Another inter-CU coordination function is downlink and uplink reusescheduling. Every TTI, the CUs can forward to the CF a list of activeUEs, separately for downlink and uplink, with the following schedulinginformation: 1) UE Identifier, 2) Queue Depth or equivalent, and 3)Scheduler Metric. The CF can perform processing using the informationreceived from all CUs and return the following scheduled UE list,separately for downlink and uplink: 1) UE Identifier, and 2) DCI. Thereuse scheduling may have low latency so that the scheduling can becompleted within 1 millisecond. In some implementations, some schedulingtasks are shifted from the CF to the CUs, e.g., to prevent the CF frombecoming the bottleneck in the scheduling process. The CF can alsocoordinate scheduling across multiple carriers as in carrieraggregation.

A third inter-CU coordination function relates to data transport. On thedownlink, each CU can form a data frame for the UEs that it is serving.In some implementations, the CUs do not form any data frames for thoseUEs that they are not serving. The RUs receive the data frames from themultiple CUs and combine the frames as needed to form the transmitteddownlink OFDM symbols. In some implementations, to reduce link rate, theCUs send unicast or narrowcast packets.

On the uplink, for each UE, its serving CU can determine the uplinkcombining set and inform the RUs accordingly. In some implementations,an RU only needs to send a UE's uplink data to the CU that is servingthat UE.

Another inter-CU coordination function involves multi-user (MU) MIMO. Onthe downlink, inter-CU coordination is implemented when UEs in theMU-MIMO are being served by different CUs. The joint precoder in MU-MIMOcan be determined by the CF. On the uplink, joint detection in a singleCU using Successive Interference Cancellation (SIC) is implemented.MU-MIMO can be implemented across CUs by allowing a CU process a UE'ssignal for SIC even when the UE is normally assigned to another CU.

Furthermore, inter-CU coordination functions are also performed inassociation with the downlink control channels. For example, the CF candetermine PDCCH reuse. The CF can form and send DCI to the CUs alongwith RU transmission strategy. The actual PDCCH packet can be formed bythe CUs. The RNTI of the DCI can determine which CU is responsible forhandling a PDCCH packet. All PDSCH and PUSCH DCIs can be handled by theCU serving the corresponding RNTI. All SI (system information)-RNTI DCIscan be handled by the SI CU. PCFICH can be sent by one CU of the cellthat is either pre-determined or selected by the CF. It is possible toimplement PHICH reuse.

A cell having multiple stacked CUs, such as the cell shown in FIG. 26can handle SRS and PUCCH as follows. For the SRS, the RUs can beconfigured to forward SRS data of the UEs only to their respectiveserving CUs. The serving CUs upload the signature vectors of theirrespective UEs to the CF.

For the PUCCH, the RUs can forward PUCCH data to their respectiveserving CUs. In some implementations, UEs in PUCCH reuse belong to thesame CU, and therefore, UEs in PDCCH reuse also belong to the same CU.

Other considerations associated with a cell having multiple stacked CUsinclude CF scalability, CF redundancy, and CF and CU stack management.In some implementations, the CF application is parallelized to increasescalability and to take the advantage of multi-core processors of theserver on which the CF is run.

A cell having multiple stacked CUs can provide all advantages of a cellhaving one CU. In addition, the cell with the multiple stacked CUs canhave a high scalability. The impact on existing CU applications issmall. The implementations may be virtualized through the server runningthe CF. The cell may be used in outdoor applications.

It is also possible to implement controller stacking without addinganother centralized node. In this case, multiple CU instances,representing same or different cells, may run on one or more hardwareplatforms, with high-speed connectivity between them (e.g., 10GEthernet) and every TTI CUs exchange information to perform schedulercoordination, for example, to serve a single cell or multiple cellsacross multiple controller instances. When coordinating across cells,such coordination may be used to support carrier aggregation or tocontrol interference at border areas between adjacent cells. Suchinformation exchange can be implemented in a one-way transmission fromeach CU instances to all other CU instances participating incoordination.

Multi-Operator Small Cell

In many enterprises and public spaces there is a need to supportmultiple operators. The example systems described herein enables suchmulti-operator deployments.

The multi-operator system uses a multi-RU mini-enclosure with a uniqueRF combining capability to allow multiple RU modules for differentoperators to plug into a single enclosure. The enclosure would beinstalled outside of view, for example above the ceiling tile, and canconnect to one or more shared external antennas via one or more antennacables. In some implementations there will be one antenna cable thatfeeds a single wideband shared external antenna. In some otherimplementations, there may be two antenna cables that feed into oneexternal wideband antenna with two antenna ports to support MIMO. Moregenerally, M cables could be feeding M SISO antennas or M/2 MIMOantennas, where M is the number of antennas utilized in the multi-RUenclosure, and each antenna has the combined signal of N RUs, and can beof any type of antenna such as omni-directional or directional. In someimplementations, the RU modules will have identical hardware. Thisallows a neutral host company that may deploy and manage themulti-operator site to reuse the same module for different operators. AnRU module can be reassigned from one operator to another operator. An RUcan also be reassigned from one frequency to another frequency, even ifsome are FDD and others are TDD.

The Multi-RU chassis can also be used for Wi-Fi by adding a Wi-Fimodule, and could also combine the Wi-Fi signal with the LTE signalsonto the external antennas. It can also be used for LTE-Advanced CarrierAggregation (CA) for a single operator. In CA, multiple carriers can beused to serve one or more UEs. In an existing deployment, additional RUmodules can be added in the form of additional carriers for the sameoperator, and the aggregation of the carriers is achieved in thecontroller.

Downlink Inter-Cell Interference Control

In some implementations, inter-cell interference on PDSCH is reducedusing hard frequency reuse (HFR). HFR can be implemented as a static orsemi-static scheme, where the available resource blocks are dividedbetween groups of cells according to K-way frequency reuse, where K istypically 3 or 7, so that each cell uses one-third (or one-seventh) ofthe available resource blocks. When only one cell transmits in eachresource block, cells in the same frequency reuse group will not see anyPDSCH interference from the others. Implementing HFR maycost(K−1)/K×100% of the available bandwidth.

Alternatively, inter-cell interference on PDSCH can be reduced usingSoft Frequency Reuse (SFR). In SFR, available resources are partitionedbetween neighboring cells in the same frequency reuse group. Differentfrom HFR where each resource block is assigned a binary state (on/off),e.g., full power or no power at all, in SFR, each resource block can beassigned any transmit power level. For example, consider the followingexample with 3 different power levels (high (H), medium (M), low (L)).Referring to FIG. 17A, in every cell 2400 a, 2400 b, 2400 c, eachresource block 2402 is assigned to one of these power levels (H, L, orM), such that in resource blocks where a cell is assigned a high power,its two neighboring cells are assigned a low power. As a result, eachcell will have two times as many low-power resource blocks as high-powerones. Each eNodeB will assign the UEs that it is serving to one of thepower levels, typically during connection set up, based on the averageSNR the UE is experiencing and possibly other factors such as the amountof data the UE has for transmission. The UEs that are in goodconditions, e.g., located near the center of a given cell, or that havelittle data to send are assigned a low PDSCH power level, whereas UEs inpoor conditions, e.g., located near the cell edge or having a lot ofdata for transmission are assigned a high PDSCH power. Accordingly, whenthe controller is serving a cell edge user, the UE will experience botha higher received signal power and a lower interference power level,boosting its average received SNR. When the UEs move and their channelconditions change, the controller can change the transmit power levelfor the UE by sending a higher layer reconfiguration message. Whenscheduling UEs for transmission on resource blocks, the controller mayeffectively need to run parallel schedulers, one per power level. Insome implementations, the strict partitioning of the resources may leadto scheduling efficiency loss, for example, due to loss of somemulti-user diversity. Such inefficiencies can become visible when thePDSCH power distribution of active UEs is mismatched relative to thepower distribution of the resource blocks. Fixed power allocation canalso be inefficient because it sometimes unnecessarily forces a lowpower transmission for a UE, even though a transmission at a higherpower level may not cause any interference to a cell edge UE served by aneighboring cell in the same frequency reuse group when the UE is on theopposite side of the neighboring cell.

Coordinated Scheduling

The efficiencies of SFR can be improved by implementing theresource/power partitioning dynamically as part of a centralizedmulti-cell scheduler in the controller. The controller can dynamicallyallocate resource blocks and transmission power based on Radio ResourceManagement (RRM) reports received from the UEs. The implementation canavoid the need to assign transmit power levels to resource blockssemi-statically as in HFR or SFR.

In LTE, each cell will periodically broadcast its NeighborList in aSystem Information Block (SIB) Type 4 (SIB4). A connected UE willmonitor the cells in the NeighborList and send Measurement Reports tothe serving cell. These reports can be sent periodically or based oncertain triggers. The reporting period and the triggers are configuredby the serving cell using an RRC-Reconfiguration message. Each UE'sMeasurement Report includes two measurements per cell: i) ReferenceSignal Received Power (RSRP) and ii) Reference Signal Received Quality(RSRQ). RSRP is the average received power of a CS-RS RE and isindicative of the received signal strength, and RSRQ is an additionalsignal quality indicator, which also provides a crude measure ofinterference. In some implementations, coordinated scheduling in thecontroller will work as described below.

Each baseband modem will send, to the central coordinator, the NeighborList RSRP reports received from each of the connected UEs it is serving,as well as the amount of data each UE has waiting for transmission.Baseband modems may send these reports upon certain event triggers, forexample when a UE is newly connected or disconnected, or when there is asignificant change in the UEs RSRP reports. It is also possible for thecentral coordinator to poll the baseband modems to get these RSRPreports.

The central coordinator will use the received information to construct abandwidth and PDSCH power allocation map for each UE and willperiodically send this information to their serving baseband modems. Anexample of the basic logic for creating this bandwidth allocation map isdiscussed below.

Individual cell modems communicate the PDSCH power allocation to theUEs, e.g., shortly after setting up the connection. For every subframe,individual baseband modems schedule UE data for transmission on PDSCH.Baseband modems schedule transmissions in a manner that is consistentwith the power levels and the bandwidth resources allocated to each UEby the central coordinator.

Next, examples using two adjacent cells are described with reference toFIG. 17B. In an example, suppose each cell 2410 a, 2410 b has oneconnected UE, and each UE has similar amounts of data waiting fortransmission. If both UEs are away from the cell boundary, the centralcoordinator would allocate the full transmission band to both UEs sinceneither would experience significant inter-cell interference. If bothUEs are near the cell boundary, then the cell coordinator would allocate50% of the transmission bandwidth to each UE at full power. If one UE isnear the cell boundary but the other is away from the cell boundary,then the cell coordinator could allocate the full transmission band toboth UEs, but assign a lower power level to the UE away from the cellboundary to reduce interference with the UE near the cell boundary inthe other cell. When the UEs have significantly different amounts ofdata waiting for transmission, the cell coordinator may give morebandwidth to the UE with more data.

In a more complex case where each cell has 10 connected UEs with 50%near the cell boundary and 50% away from the cell boundary and UEs nearthe cell boundary have similar amounts of data as the UEs away from thecell boundary, central coordinator could allocate resources as follows:UEs that are away from the cell boundary are allocated the fulltransmission bandwidth, but at a reduced power level and UEs near thecell boundary are allocated 50% of the transmission band in anon-overlapping manner, but at full power.

If the ratio of the number of UEs at the cell edge to the number of UEsat the cell center is different from 1:1 or the amount of data the UEshave for transmission near the cell edge is different from the amount ofdata the UEs have for transmission at the cell center, the centralcoordinator can adjust the bandwidth and power allocation process(es) tomatch the data needs of the UEs. In some implementations, theadaptability of the allocation can make the system significantly morebandwidth-efficient, while improving the cell-edge performance fordisadvantaged UEs.

In some implementations, there may be interference between the radionetwork and other networks, such as the macro network, and suchinterference may also be considered and reduced. Release 8 supportsmessages in the X2 interface to allow eNodeBs to exchange information onpower levels that are used in each of the resource blocks of theeNodeBs. In some implementations, the X2 interface is used between thecontroller of the disclosure and eNodeBs of the other radio networks(e.g., macrocells). The user can facilitate exchange of informationbetween the controller and the eNodeBs to support coordinatedscheduling. As an example, each eNodeB can indicate to the controllerfor each resource block whether the power level in that resource blockwill remain below a certain threshold, which is also separatelysignaled. This will allow the controller to schedule those UEs locatedat cell edges in resource blocks where the neighboring cells aretransmitting below a certain power level. Similar techniques can be usedto coordinate transmissions by different controllers in the same radionetwork, in which each controller can be informed about the SFR (SoftFrequency Reuse) power assignments via a management system or using avariant of the X2 interface.

Interference Control Techniques for Release 10 UEs

In some implementations, inter-cell control channel interference forhierarchical networks with closed access or range extension can bereduced by having the cells turn off (blank) power in all resourceblocks in certain subframes. When no PDSCH data is transmitted in asubframe, there is also no control messages sent on the downlink controlchannel, which significantly reduces PDCCH interference. In someimplementations, by configuring these blank frames as so-called MBSFN(Multicast/Broadcast Subframes), one can also reduce (e.g., eliminate)interference from CS-RS REs in the PDSCH region.

In an MBSFN subframe, CS-RS is only transmitted in the control region ofthe subframe. This reduces (e.g., eliminates) the CS-RS interferenceinto PDSCH (although not necessarily to PDCCH) transmissions inneighboring cells. MBSFN subframes in LTE were developed in Release 8 tocarry broadcast/multicast signals, but they can also be used to send nodata at all. A cell can be configured to send MBSFN subframes accordingto a certain pattern, and the pattern can be communicated to UEs via theSystem Information Block (SIB). In some implementations, only 6 out of10 subframes (e.g., #1, 2, 3 and 6, 7, 8) in a radio frame can be usedfor MBSFN. MBSFN frames have a control region of up to 1 OFDM symbol for1 or 2 TX antennas and 2 OFDM symbols for 4 TX antennas.

Using blank MBSFN subframes alone may not eliminate inter-cellinterference between PBCH, system information (SIB) and PSS/SSStransmissions. In some implementations, the inter-cell interference isbetween a small cell and a single macro cell, and the interference canbe reduced or eliminated by offsetting the subframe numbering in thesmall cell relative to the macro cell. For example, if the relativesubframe number of the small cell network has an offset of 2 relative tothe macrocell network (e.g., subframe #0 in small cell network coincideswith subframe #2 in the macrocell network), and macrocell subframes 2and 7 are ABS/MBFSN subframes, small cell UEs can receive PSS/SSS andPBCH without any interference from the macrocell.

In some implementations, the macro cell coordinates its transmissionsonly with the controller and it is not necessary for the macro celleNodeB to coordinate its transmissions with multiple base stations orRUs.

Coordinated MultiPoint (CoMP)

CoMP refers to techniques that involve coordination between neighboringcells to reduce the effects of inter-cell interference. Full-blowncoordination is referred to as Joint Transmission (JT). In JT, two ormore baseband modems cooperate to serve their UEs via all RUs that theyjointly control. All available antennas can be used to serve one UE withSingle-User MIMO or multiple UEs simultaneously using Multi-User MIMO.In some cases where JT is implemented, UEs send CSI feedback not onlyfor the antenna ports of their serving cell, but also for antenna portsof neighboring cells.

In JT, similar to the single-cell multi-user MIMO, transport blocks fordifferent UEs may be processed in parallel and then combined before theIFFT. However, different baseband modems handle the processing oftransport blocks of UEs in different cells. In some implementations, thecontroller may include a coordination unit for coordinating schedulingindifferent baseband modems. The coordination unit may also serve as anaggregation point for combining processed transport blocks originatingin different baseband modems. As an example, a radio network 2700 shownin FIG. 20A includes three cells formed by baseband modem 2706 andremote unit(s) 2716, baseband modem 2708 and remote unit(s) 2718, andbaseband modem 2710 and remote unit(s) 2720. The controller 2704controlling the three cells includes a coordination unit 2702, thatserves as an aggregation point for combining (represented by the symbol“s”) transport blocks originating from different modems 2706, 2708,2710.

Alternatively, as shown in FIG. 20B, in a radio network 2730, basebandmodems 2732, 2734, 2736 controlling cells that including remote unit(s)2742, remote unit(s) 2744, remote unit(s) 2746, respectively, maydirectly exchange data among themselves so that each baseband modem cancombine all signals destined to the UEs (not shown) they serve.

In some implementations, referring to FIG. 20C, in a radio network 2760,each baseband modem 2762, 2764, 2766 sends processed transport blocks tothe RUs 2772, 2774, 2776 and the RUs perform the combining beforeapplying the IFFT.

A somewhat reduced CoMP capability is called Dynamic Point Selection(DPS). In DPS, the serving cell sends PDSCH transmission on atime-frequency resource via only one cell TX antennas based on feedbackcell selection received from the UE. The selected cell can be varieddynamically from one subframe to the next, and even between resourceblocks within the same subframe. The selected cell may be different fromthe serving cell of the UE.

Another form of CoMP is Coordinated Beamforming (CB). In CB, when aserving cell is transmitting to a UE from its RUs, it also accounts forinterference it will be creating for another UE in a neighboring cell.By choosing the precoding vector(s) to null the interference to theneighbor cell UE, the controller allows the baseband modem of aneighboring cell to serve the other UE at a higher data rate.

Release 11 has new capabilities to support coordinated transmission. Forexample, Release 11 allows UEs to report CSI for multiple CSI-RS, whichmay belong to different cells.

Communications Between the Controllers and the Remote Units

As explained previously, the CUs and the RUs of a radio network areconnected through a switched Ethernet network (see, e.g., FIG. 3). Insome implementations, the interface between the CUs and the RUs willcarry time-domain IQ symbols (sometimes also referred to as signals) inEthernet frames. However, the bit rate of the time-domain IQ symbols maybe too high for an Ethernet network. In some implementations, instead ofsending the time-domain IQ symbols a compressed representation of thetime-domain IQ symbols is sent to reduce the bit rate and to provide adata rate between the CUs and the RUs that is compatible with thelow-cost switched Ethernet network. In some implementations, on thedownlink, the CUs of the radio network send the IQ symbols when they arein the frequency-domain and prior to performing the IFFT (inverse fastFourier transform) on the frequency-domain IQ symbols. A CU sends thefrequency-domain IQ data representing each OFDM symbol to an RU, forexample, by quantizing the real and imaginary components of thefrequency-domain symbols. The quantizer output bits are then packetizedin Ethernet frames and transmitted to the RUs over the Ethernet network.The RU reconstructs the quantized frequency-domain IQ symbols beforeapplying the IFFT, inserting a cyclic prefix and performing thefiltering, modulation and RF processing.

For the purpose of discussion, a radio network for a 10 MHz FDD LTEsystem is used as an example. For each TX antenna port, each OFDM symbolhas 600 subcarriers and there are 14 OFDM symbols in every 1 mssubframe. Each subframe has 8,400 Resource Elements (REs) in total. EachRE corresponds to one subcarrier in one OFDM symbol. On the downlink,the first 1-3 OFDM symbols in a subframe are primarily used for controlsignaling (e.g., PDCCH, PHICH, and PCFICH) and the remaining OFDMsymbols carry primarily user data on the shared data channel (PDSCH).Reference signals and other common channels are spread across thetime-frequency axis.

Compressing the IQ symbols in the frequency domain can reduce the bitrate of the traffic sent over the Ethernet network. The compressedfrequency-domain IQ symbols are transmitted over the Ethernet networkwithout guard band zeros or any cyclic prefix. When the CU uses a 12-bitquantizer to compress the frequency-domain IQ symbols, the nominal bitrate of the frequency-domain IQ stream is about 403 Mb/s for 2 TXantennas and 806 Mb/s for 4 TX antennas. This represents a 45% reductionin bit rate compared to quantizing the time-domain IQ stream using thesame quantizer (735 Mb/s for 2 TX antennas and 1471 Mb/s for 4 TXantennas). The rate between the CU and the RUs is reduced and the CU andthe RUs are allowed to communicate through Ethernet links operating at aspeed in the order of Gb/s with less latency.

On the uplink, in addition to RF processing and demodulation, the RUsremove the cyclic prefix from the time-domain IQ samples for eachreceived OFDM symbol and apply the FFT to produce the frequency-domainIQ symbols. The information carried by the symbols is then quantized,packetized in Ethernet frames, and transmitted to the CU over theEthernet network. When the 12-bit quantizer is used, the resulting bitrate of the frequency-domain IQ symbols on the uplink is substantiallythe same as that discussed for the downlink.

Described below are example techniques that may be used to reduce thedata rate between the CU and the RUs.

Downlink Compression within a Cell

In some implementations, all antennas of the RUs that belong to the sameantenna port in the same cell (unless explicitly specified as a virtualcell, the cells are physical) transmit the same LTE signal. Accordingly,on the downlink, for each antenna port the CU sends the samefrequency-domain IQ symbol to each RU in the cell. The frequency-domainIQ symbols that the CU needs to send to the RUs include the CS-RS andCSI-RS reference signals, the control channels PDCCH, PCIFCH and PHICH,the shared data channel PDSCH, and the common channels PBCH and PSS/SSS.

In some implementations, the CU performs a simple form of compression bybroadcasting the frequency-domain IQ symbols to all RUs in the cellusing broadcast Ethernet frames. To implement the broadcast, all RUs inthe same cell are configured to belong to the same VLAN (virtual localarea network). The CU sends to its nearest Ethernet switch an Ethernetbroadcast frame that carries an ID of the VLAN. The Ethernet switch inturn sends the Ethernet broadcast frame to all the RUs on the VLAN thatare directly attached to the Ethernet switch and to other Ethernetswitches that provide paths to other RUs on the same VLAN. In suchimplementations, traffic load on the Ethernet switches on the downlinkdoes not grow with the number of RUs that belong to the same cell.

The broadcast on the Ethernet network and the implementation of theVLANs can simplify processing complexity and reduce the data ratebetween the CU and the Ethernet network. The reduction in the data rateis desirable to reduce the Ethernet frame size and latencies at theswitches.

For the purpose of discussion, the previously introduced example of aradio network implementing the 10 MHz FDD LTE with 2 TX antennas is alsoused as an example in the discussion below. The 8,400 frequency-domainIQ symbols in each 1 ms subframe are organized in the form of a resourcegrid that has 600 OFDM subcarriers in 14 OFDM symbols. The 14 OFDMsymbols are split into two time slots each having a length of 0.5 ms.Each time slot is further split into 50 PRBs (physical resource blocks),each containing 84 frequency-domain IQ symbols arranged in the form of a7×12 grid. In some implementations, each PRB carries at most one PDSCHmixed with reference signals, such as the CS-RS and the CSI-RS. The PRBscan also carry one or more LTE downlink control channels PDCCH, PHICH orPCFICH, or the common channels PSS/SSS and PBCH, mixed with the CS-RSand the CSI-RS.

The downlink frequency-domain IQ symbols are discrete-amplitude symbolschosen from a signal constellation. The PSS/SSS is carried onfrequency-domain IQ symbols that lie on a circle. The PDCCH, PCFICH,PBCH, CS-RS, CSI-RS and DM-RS are carried on frequency-domain IQ symbolschosen from a QPSK/BPSK signal constellation. Without precoding, thefrequency-domain IQ symbols that carry the PDSCH are chosen from a QPSK(quadrature phase-shift-keying), 16-QAM (quadrature amplitudemodulation), or 64-QAM signal constellation. The PDSCH modulation orderis chosen based on the signal quality reported by a UE. In the presenceof precoding, the frequency-domain IQ symbols that carry PDSCH are basedon the product of a precoding matrix with an input vector, whosecomponents are symbols chosen from a QPSK, 16-QAM, or 64-QAMconstellation.

The CU can choose downlink frequency-domain IQ symbols directly from adiscrete-amplitude QAM constellation or by applying a matrix operationto symbols chosen from a discrete-amplitude QAM constellation. Theaverage energy of the frequency-domain IQ symbols can vary betweendifferent downlink channels, but is fixed for a given channel within aResource Element Group, or REG (for control channels) or a PRB (forPDSCH). A REG is a group of 4 consecutive REs in an OFDM symbol. In someimplementations, the PDSCH on the 4^(th) OFDM symbol of the slot canhave a different average energy level from those fixed average energylevels.

Methods of Compressing the Frequency-Domain IQ Symbols

The symbols transmitted between the CU and the RUs can be compressed invarious ways. In the discussion below, the first three methods, MethodsI, II, and III, are based on quantization, and the fourth method, MethodIV, is based on modulation-level compression.

Fixed Quantization

In this method, the frequency-domain IQ symbols are quantized using afixed uniform scalar quantizer having a fixed rate R and a fixed stepsize Δ. The step size is selected by the CU based on the expectedprobability distribution of the frequency-domain IQ symbols. Inimplementations, the CU quantizes the real and imaginary components ofthe frequency-domain IQ symbols serially and transmits the binary datarepresenting the quantized IQ symbols for each TX antenna to the RUs.The values of R and Δ are sent to the RUs when the RUs initially connectto the CU. The RUs use the information about the rate R and the stepsize Δ to reconstruct the frequency-domain IQ symbols based on the datareceived from the Ethernet network. In some implementations, when thereis a major change in configuration of the radio network that changes Rand/or Δ, the CU sends the modified R and/or Δ to the RUs. In theexample with the 10 MHz FDD LTE having 2 TX antennas per RU and a fixed12-bit quantizer, the quantized frequency-domain IQ stream has a datarate of 403 Mb/s between the CU and the RUs.

Adaptive Step-Size Quantization

Instead of applying a fixed quantizer step size Δ, in this examplemethod, the step size is dynamically varied based on the average energylevels of the downlink channels, which can be different for differentchannels. Dynamically adjusting the quantizer step size can reduce theaverage mean-squared quantization errors for a given bit rate R of thequantizer. In some implementations, the dynamically adjusted step sizecan also be used to reduce the quantizer rate R without increasing thequantization error.

Information about the dynamically adjusted quantizer step sizes iscontained in side information that a CU sends to the RUs. The RUs canreconstruct the quantized frequency-domain IQ symbols based on the stepsize information. In some implementations, the CU sends some sideinformation to the RUs once per subframe, and the other side informationonce per-REG or once per-PRB. At the beginning of each subframe, the CUsends side information that contains information about the position ofthe CS-RS and the CSI-RS, the step size associated with the CS-RS andthe CSI-RS, and the length of the control region. In someimplementations, the information about the actual step size of thequantizer is sent before each REG (in the control region) or beforesending any PDSCH data in each PRB (in the PDSCH region). The PDSCHenergy levels can be different in the 4^(th) OFDM symbol of a time slot.Accordingly, two step sizes can be sent per PRB. The transmission ofside information can be distributed across the subframe evenly to reducethe peak data rate. When each step size is represented by a 12-bitindex, the side information takes less than 5 Mb/s of link capacity.

In some implementations, the same step size is used for both TX antennasof a RU to limit the amount of side information. In otherimplementations, the step sizes for the two TX antennas can bedifferent.

The rate R of the quantizer is chosen so that the quantization noisedoes not impact the UE's receiver performance, including when the mostdemanding (e.g., most noise-sensitive) MCS (modulation and codingscheme) is used in PDSCH. In some implementations, a 9-bit or 10-bitquantizer delivers an SQNR (signal-to-quantization noise ratio) in therange of 50-60 dB, which is more than 20 dB higher than the target SINR(signal-to-interference-plus-noise ratio) required for uncoded 64-QAM. Aquantizer rate of 9-10 bits can produce a maximum data rate of 302-336Mb/s, which represents a 17-25% compression relative to the maximum datarate in Method I.

Adaptive Rate and Step Size Quantization

In a third compression method, both the rate R and the step size Δ ofthe quantizer are dynamically adjusted based on the quantization noisetolerance of each downlink channel. Dynamically varying the quantizerrate R can reduce the average data rate but does not reduce the peakdata rate, and the reduced average data rate can reduce the averagepacket length and the latencies at the Ethernet switches.

The relationship between the quantizer rate R and the performance of thedownlink channel is explained below using an example scenario where eachUE has one RX antenna and each RU has one TX antenna. The discussionsand the calculations can be readily extended to UEs and RUs that havemore than one antenna. In the example, the frequency-domain IQ symbol rreceived by the UE can be written as:r=(s+q)×h+i+w,where s represents a complex-valued frequency-domain IQ symbol having anaverage energy E_(s), h is the corresponding complex-valuedfrequency-domain channel gain, q is the quantization noise, and i and wrepresent the received interference and thermal noise, respectively. Thesignal-to-quantization noise ratio of the quantizer, SQNR, is defined tobe E_(s)/E_(q), where E_(q) is the average energy of the quantizationnoise.

The signal to interference plus noise (SINR) ratio received at the UE isdenoted as SINR′ and can be written as:

$\begin{matrix}{{SINR}^{\prime} = {E_{s} \times {{h}^{2}/\left( {E_{i} + E_{w} + {E_{q}{h}^{2}}} \right)}}} \\{{= {{SINR}/\left( {1 + {{SINR}/{SQNR}}} \right)}},}\end{matrix}$where SINR=E_(s)×|h|²/(E_(i)+E_(w)) is the SINR received at the UE inthe absence of any quantization noise, E_(i) is the energy of theinterference noise, and E_(w) is the energy of the thermal noise. Basedon the equation for SINR′, when SQNR>>SINR, SINR′≈SINR. In other words,in this example, the quantization noise does not have a substantial ornoticeable impact on the performance of the signal received at the UEwhen SQNR>>SINR.

In this example, the SQNR increases with the quantizer rate R, e.g., byabout 6 dB for every increment of R by 1 when R is large. IfSINR_(target) represents the desired SINR required at the UE for a givenMCS (modulation and coding scheme) for reliable reception, implementingthe quantization does not cause SINR′ to drop noticeably below theSINR_(target) when the quantizer rate R is chosen such thatSQNR>>SINR_(target). Accordingly, when the target SINR for a modulationformat is low, the rate R (e.g., the accuracy) of the quantizer can bereduced.

In some implementations, the quantizer rate R for PDSCH transmissionwill be the highest for PDSCH MCS of 28 and will be the lowest for PDSCHMCS of 0, which respectively correspond to the most and least demanding(in terms of noise sensitivity) modulation and coding schemes currentlysupported in the LTE standard. In the control channels, the underlyingmodulation format is QPSK/BPSK and a relatively low quantizer rate R canbe used. In some implementations, when a relatively low quantizer rateis used, the SINR received at UEs having good channel conditions can bereduced by the quantization noise. However, the reduced SINR does notsubstantially affect the performance of the UE when the reduced SINR isabove the target SINR.

Similar to Method II, the CU sends side information that containsinformation about the step size of the quantizer to the RUs to help theRUs reconstruct the frequency-domain IQ symbols from the received databits. In addition, the CU also dynamically sends the quantizer rate R tothe RUs for each REG and PRB and for the reference signals CS-RS andCSI-RS. Dynamically varying the quantizer rate and step size can reducethe quantization noise caused by a fixed average quantizer rate.Alternatively, when a certain average amount of quantization noise ispermissible for the signal transmissions, the average quantizer rate canbe reduced when the quantizer rate is dynamically adjusted instead ofbeing fixed.

In addition to compressing the symbols being sent to the RUs, the CU canfurther reduce the average data rate between the CU and the RUs by notsending any data for unused REGs or PRBs. For example, when only 50% ofthe REGs and PRBs in a time slot are in use, e.g., carrying data, theaverage data rate is further reduced by 50%.

When multiple TX antennas are used, the same quantizer rate and stepsize can be used for all antennas of each RU so that the amount of sideinformation does not grow with the number of TX antennas. In someimplementations, the quantizer rate and the step size can be differentfor each antenna and the average quantizer rate is then further reduced.

In the examples of the quantizers in Methods I-III, a scalar uniformquantizer is used because of its ease of implementation. However, thesemethods are equally applicable to other types of quantizers, such asnon-uniform scalar quantizers, vector quantizers, etc. The step size andthe rate of the quantizer are varied to adapt the quantizer to thecharacteristics of the quantized symbols. It is also possible to varyother parameters of the quantization process, such as the gain of thequantizer input.

Modulation-Level Compression

In this fourth example compression method, the CU sends thefrequency-domain IQ symbols in the form of binary data based on thestructure of the frequency-domain IQ symbols known to the CU and withoutimplementing any quantization. As discussed previously, thefrequency-domain IQ symbols belong to a discrete-amplitude signalconstellation, or they can be derived by transforming modulation symbolschosen from a discrete-amplitude signal constellation. By sending binarydata representing the discrete-amplitude signals along with sideinformation required to apply any required transformations, thecontroller can avoid quantization noise.

In use, the CU sends the binary data representing the modulation symbolsto the RUs one OFDM symbol at a time in the same order as the symbolsare to be transmitted by the RUs over the air. In particular, the binarydata that represents the control channels is sent in groups of REGs, andthe binary data that represents the shared data channels is sent ingroups of 12-symbol blocks that belong to the same PRB. Furthermore, atthe beginning of each time slot, the CU sends some portions of sideinformation to the RUs. Other portions of the side information are sentat the beginning of each REG in the control region and before sendingthe data in the first PDSCH OFDM symbol of that time slot. The RUs parsethe received data and reconstruct the frequency-domain IQ symbols basedon the side information.

In this method, some of the baseband modem transmitter functions areimplemented in the CU and some other baseband modem transmitterfunctions are implemented in the RUs. For example, the forward-errorcorrection function is implemented in the CU, whereas the precoding andthe IFFT functions are implemented in the RUs. The downlink processingcan be partitioned between the CU and the RU in many other ways. In someimplementations, it is even possible to move the entire downlink modemprocessing to the RU. In this case the controller sends all necessarydata, including the transport block data, to the RU along with allnecessary side information. In some implementations, this will reduce,e.g., minimize, the data rate between the controller and the RUs, butmay increase the amount of processing in the RUs. In some cases, theinterface between the controller and the RUs is implemented using aso-called FAPI (Femto Application Platform Interface) developed by theSmall Cell Forum, except that the FAPI will be implemented over anEthernet network.

In some implementations that use downlink carrier aggregation, forexample, by aggregating licensed carriers or a combination of licensedand unlicensed carriers, the RUs may implement the downlink Layer 1functions for multiple carriers. In such examples, interface between thecontroller and the remote unit may support the transmission of PDSCHtransport block data and control information for multiple carriers.

Described below is the representation of frequency-domain IQ symbols bybinary data for each type of downlink channel.

CS-RS Reference Symbols

The CS-RS reference symbols are complex-valued binary symbols chosenfrom a QPSK constellation, whose gain may remain constant during thesubframe. When each RU has multiple TX antennas, the CS-RS referencesymbols also include “zero” symbols to avoid interference between theantennas. The CS-RS reference symbols on different antennas differ onlyin their relative positions on the resource grid (see, e.g., grids 730,740 of FIG. 7). The CU includes in the side information a 3-bit index torepresent the CS-RS frequency shift and a 12-bit number to represent thegain. The side information is sent to the RUs at the beginning of eachsubframe, through which the RUs learn about the positions of all CS-RSreference symbols in the resource grid for all TX antennas, except for afixed frequency index offset between 0 and 5. The frequency index offsetdepends on the Cell-ID. Based on the frequency index offset, the RUs candetermine the position of the zero REs, for which no data bits need tobe sent. For the nonzero CS-RS REs, two bits are sufficient to representeach CS-RS symbol. The RUs receiving the binary data, two bits for eachRE, can reconstruct the IQ symbol by inserting the correctcomplex-valued CS-RS symbols and the “zero” REs into the resource gridfor each TX antenna based on the side information.

CSI-RS Reference Symbols

The CU can handle the CSI-RS symbols used in Transmission Mode 9 ofRelease 10 similarly to the CS-RS reference symbols discussed in section(i). At the beginning of each subframe, the CU sends to the RUs sideinformation to indicate the position of the CSI-RS symbols in theresource grid. The side information can be based on parameters such asCSI configuration, ZeroPower CSI-RS Index, scale factor, etc. Using theside information and the data received from the CU, which is two bitsfor each RE, the RUs can insert the correct complex-valued CSI-RSsymbols and the “zero” REs into the resource grid for each TX antenna.

Control Symbols

The frequency-domain IQ symbols in the control region (e.g., thedesignated first 1-3 OFDM symbols) that are not used by CS-RS belong toPCIFCH, PHICH or PDCCH. In some implementations, the control symbols arerepresented by binary data on a per REG basis. Each REG has 4 REs thatare contiguous, except for the CS-RS reference symbols inserted inbetween. Each control channel is carried in multiple REGs that arespread in frequency (e.g., the REGs are located in different parts ofthe transmission frequency band). For each REG, the CU sends sideinformation to the RU for the RU to parse the received binary data. Theside information is sent per REG and may include 2-bit data to representthe channel type (e.g., PDCCH, PCFICH, PHICH, or unused) and 12-bit datato represent channel gain. At the beginning of each subframe, the CUsends to the RU 2-bit side information to indicate a length of thecontrol region. In some implementations, to process the receivedsignals, the RUs do not need to know in advance the location of thedifferent control channels in the control region.

When each RU has multiple TX antennas (e.g., N TX antennas, where N isan integer larger than 1), the radio network transmits the controlsymbols using Alamouti TX diversity. In implementations, the CU sendsthe 16-bit binary data that represents the 4 QPSK (quadrature phaseshift keying) symbols in each REG to the RU. The RU implements signchange and conjugation operations for TX diversity to generate the 4×NQPSK symbols that represent the frequency-domain IQ symbols in the REGfor all N TX antennas.

The PHICH can be represented by binary data based on the fact that thetransmitted frequency-domain symbols for PHICH are also chosen from adiscrete signal constellation. Each PHICH represents 1-bit of ACK/NAK(acknowledgement/negative acknowledgement) information for uplink HARQ(hybrid automatic repeat request). The PHICH bit is encoded into acomplex-valued 12-symbol sequence chosen from a binary BPSK signalconstellation with a 45 degree rotation. The CU can transmit binary datarepresenting up to 8 PHICH bits together in a PHICH group. For thetransmission, the complex-valued symbols representing all PHICH bits inthe PHICH group are summed together to obtain 12 complex-valued PHICHgroup symbols. As can be seen these symbols are chosen from adiscrete-amplitude constellation. The real and imaginary components ofthe 12 complex-valued PHICH group symbols can each be represented by aninteger in the interval [−6, 6], together with a gain that may remainconstant during the subframe. The 12 complex-valued PHICH group symbolsare mapped to 3 REGs, e.g., in the first OFDM symbol of the controlregion after applying the TX diversity on a per REG basis. The CU sendsto the RUs a gain value represented by a 12-bit index, followed by 8-bitdata that represents the real and imaginary components of eachcomplex-valued PHICH group symbol before applying the TX diversity. TheRUs can use the received information to apply the TX diversity andreconstruct the frequency-domain IQ symbols for all TX antennas.

In some implementations, the PHICH symbols can also be transmitted usinga 16-bit representation of the real and imaginary components of thefrequency-domain IQ symbols for each antenna. Compared to the 8-bitrepresentation, the data rate between the CU and the RUs for the 16-bitrepresentation is higher; however, the RUs can reconstruct thefrequency-domain IQ symbols in a simpler way.

PCFICH and PDCCH can also be readily represented by binary data andtransmitted from the CU to the RUs. In particular, each REG for PCFICHor PDCCH carries 4 QPSK symbols, which are sent on multiple TX antennasusing Alamouti TX diversity. The CU sends 2 bits of data per RE, or 8bits of data per REG to the RUs, which represent the modulated symbolsbefore TX diversity.

PDSCH Symbols

Most of the REs in the OFDM symbols that are outside the control regionare used by PDSCH, except that the PBCH uses the middle 72 subcarriersin the first 4 OFDM symbols in the first time slot of every 10 ms radioframe, and that the PSS/SSS uses the middle 72 subcarriers in the last 2OFDM symbols in time slots 0 and 10 of every 10 ms radio frame. ThePDSCH symbols for single-antenna transmission are complex-valued and arechosen from a QPSK, 16-QAM or 64-QAM constellation, which can berepresented by 2, 4 or 6 bits of data, respectively. The gain of a givenPDSCH symbol may remain constant during the subframe (except possibly inthe 4^(th) OFDM symbol of each time slot), and the gain for differentPDSCH channels can be different. Resources assigned to each PDSCH are inone or more consecutive VRBs (virtual resource blocks) and can be mappedto PRBs in a localized (consecutive) or distributed (non-consecutive)manner. In some implementations, the CU assumes that the PDSCH changesat every PRB boundary, and sends side information to the RUs on a perPRB basis. The update of side information on a per PRB basis cansimplify the operation of the RUs in reconstructing the PDSCH symbols.In other implementations, localized resource allocation is used and theCU sends side information on a per channel basis, which is less frequentthan sending the side information on a per PRB basis.

For the purpose of discussion, it is assumed that the CU sends theper-PRB side information before sending the first OFDM symbol of thetime slot outside the control region. The side information includes a1-bit index that indicates whether or not PDSCH symbols are present fortransmission and another 1-bit index that indicates the presence ofPSS/SSS in even-numbered time slots or the presence of PBCH inodd-numbered time slots. The side information also includes a 2-bitindex that represents the modulation order (BPSK for DM-RS, QPSK, 16-QAMor 64-QAM), a 4-bit index that represents the PDSCH transmission mode(e.g., FIG. 8, TM #1-9), and an index representing the precodingcoefficients or a 16-bit representation of each complex-valued precodingcoefficient (TM #9). The side information is followed by binary datarepresenting the PDSCH modulation symbols.

The RUs use the side information to complete the baseband modemoperations and to generate the frequency-domain IQ symbols. In theimplementations where the PDSCH uses Transmission Mode 9, thedemodulation reference symbols (DM-RS) can also be viewed as QPSKsymbols using the same gain as the PDSCH symbols. Accordingly, nospecial treatment may be required for the REs of DM-RS.

In the previously discussed example in which a radio network implementsthe 10 MHz FDD LTE, there are 50 PRBs in each 0.5 ms time slot. EachOFDM symbol that carries no CS-RS has 12 PDSCH REs in each PRB, whereasthe OFDM symbols that carry CS-RS have 8 PDSCH REs per PRB (assumingthat there are 2 TX antennas). A PRB that carries PBCH has 32 REs forthe PDSCH.

When multiple antennas are in use for a PDSCH, the CU can reduce theamount of data that needs to be sent to the RUs based on the knowledgeof the underlying structure of the multiple-antenna transmitter. Thefrequency-domain IQ symbols in TX diversity are chosen from a QAMconstellation, and at least some of these IQ symbols are dependent oneach other. For example, a group of N² frequency-domain IQ symbolstransmitted on N TX antennas can be derived from N input modulationsymbols, which are chosen from a discrete-amplitude complex-valuedconstellation, using operations such as sign changes or complexconjugations. Accordingly, instead of sending information for N×N=N²frequency-domain IQ symbols, the CU can send information for the N inputmodulation symbols and indicate that TX diversity is used. The RUs canimplement the TX diversity operations to produce the N² symbols fortransmission in N subcarriers on N TX antennas. As a result, the datarate between the CU and the RUs does not increase when the number of TXantennas is increased.

In general, the frequency-domain IQ symbols for an N-antenna MIMOtransmitter can be written as:Y=PX,where X is a K-dimensional PDSCH input vector whose components arechosen from the underlying QAM signal constellation, P is an N×Kprecoding matrix, and K is the number of layers being transmitted.Instead of quantizing Y as if it were some continuous random vector, theCU sends data bits that represent the K modulation symbols in the vectorX along with the precoding matrix. The precoding matrix does not varywithin a subframe, and, in some implementations, the CU only sends theprecoding matrix once per PRB instead of once every OFDM symbol.

For Release 8 closed-loop MIMO, the precoding matrix is chosen from afixed set and the precoding matrix can be represented by a shortprecoding index of a few bits. In the transmission Mode 9 of Release 10,less than 64 bits are needed to represent the precoder coefficients (16bits per complex coefficient) (assuming that there are 2 TX antennas).

The data rate for the frequency-domain IQ symbols can be significantlyreduced when the number of layers K is less than the number of the TXantennas N. The data rate increases with the number of layers. However,even when K=N (e.g., full-rank spatial multiplexing), sending binarydata representing the QAM modulation symbols instead of sending theprecoded frequency-domain IQ symbols can reduce the data rate and avoidquantization noise. To transmit K layers, the data rate for the PDSCHinput data is K times the data rate for a single-layer.

Other Symbols

The CU can readily handle the binary representation of symbols on theother downlink common channels. For example, PBCH REs can be treatedsimilarly to PDSCH using QPSK modulation and TX diversity. The CU canuse 1 bit of side information to indicate the presence or the absence ofthe PBCH in odd time slots. In some implementations, the REs that carrythe synchronization symbols PSS/SSS are sent without any compression as16-bit integers to represent the real and imaginary components of thefrequency-domain IQ symbols. Similarly, 1 bit of side information can beused to indicate the presence or the absence of PSS/SSS in even timeslots.

Summary

In the methods described above, the downlink baseband modem functionsare split between the CU and RUs in such a way that reduces the datarate on the Ethernet network, while keeping the processing complexityvery low at the RUs. For example, using the specific partitioningdescribed above, the bit rate on the Ethernet network can be reduced toaround 100 Mb/s for two transmit antennas and two layer PDSCHtransmission. Actual data rate will be even lower when the airlinkresources are not 100% utilized. In addition to a lower bit rate, themethod also eliminates quantization noise altogether. Other ways ofpartitioning the data between the CU and the RUs are possible. Forexample, it is possible for the RUs to perform all the physical layerfunctions, while the scheduling and higher-layer processing is performedin the CU.

Uplink Compression within a Cell

In some implementations, the LTE uplink in the example radio networksdescribed herein is different from the downlink in many ways. Forexample, the uplink signals received by different RUs in the same cellare not identical. The different uplink signals can have differentchannel gains, noise and interference levels which can be exploited bythe controller for power and diversity gains. However, when a cellcontains multiple RUs and all RUs send their received signals to the CU,the CU receives a larger amount data on the uplink than it broadcasts tothe RUs on the downlink.

Similar to the techniques used in downlink compression, the techniquesfor uplink compression also take into account one or more of thefollowing additional differences between uplink and downlink. First, onthe uplink, without full-blown demodulation and decoding, the RUs cannotknow precisely the discrete-amplitude modulation symbols transmitted bythe UEs.

Second, the modulation format on the LTE uplink, SC-FDMA (single carrierfrequency division multiple access), is different from the OFDMA schemeused on the downlink. Instead of using the modulated symbols or theirprecoded versions as frequency-domain IQ symbols, the modulation symbolsin SC-FDMA are time-domain signals. These time-domain signals aretransformed by the UE into frequency-domain IQ symbols using a DFT(Discrete Fourier Transform). Compared to the symbols on the downlink,the frequency-domain IQ symbols obtained from the DFT transformation canexhibit a less uniform and more like a truncated Gaussian statistics,especially when the UE is assigned many RBs.

On the uplink, resources in a PRB are allocated on a contiguous manner,and frequency hopping may be utilized between two time slots of asubframe. As an example, the PUSCH PRBs (with DM-RS in the middle)assigned to a UE are consecutive and can hop between slots 0 and 1 witha known gap between them. The 4^(th) OFDM symbol of each assigned PUSCHPRB is DM-RS. The SRS, if present, is transmitted in the last symbol ofthe subframe, e.g., at every other subcarrier. The PUCCH transmissionsinclude QPSK symbols modulating a complex-phase sequence and anorthogonal cover transmitted over two PRBs at the opposite edges of aband. In some implementations, multiple UEs can transmit PUCCH signalson the same PRBs in the same subframe. The first L (which is an integer)PRB pairs carry CQI/PMI/RI transmissions, possibly together with HARQACK/NAKs, using Format 2. Additional PRB pairs are available for HARQACK/NAKs and scheduling requests.

Referring to FIG. 8, for PUSCH transmission, a UE 1204 modulates 1200and transforms 1210 time-domain symbols x 1202 into frequency-domainsymbols s 1203, performs a resource mapping 1212, and then performs afull IFFT 1214 to generate the time-domain signals for transmission overthe air to the RUs. One or more RUs 1206 in a cell receive thetransmitted signals through one or more channels 1208 via its antennas,apply RF processing to generate the received time-domain IQ signals, andapply an FFT 1220, 1222 to produce the received frequency-domain IQsignals r₁ 1216, r₂ 1218.

Assuming that a cell includes K RUs, where K is a positive integer, andthat the k^(th) RU has two antennas for receiving signals (RX antennas)from a UE that has one TX antenna for transmitting the signals, thefrequency-domain IQ symbol, r_(ki), received at the l'^(th) RX antenna(l=1 or 2) of k^(th) RU in some fixed frequency position in an OFDMsymbol can be expressed in the following forms:r _(k1) =s×h _(k1) +i _(k1) +w _(k1),r _(k2) =s×h _(k2) +i _(k2) +w _(k2),where s is the frequency-domain IQ symbol transmitted by the UE (see,e.g., FIG. 8), h_(k1) and h_(k2) are the channel coefficients, i_(k1)and i_(k2) represent interference from UEs in other cells, w_(k1) andw_(k2) are thermal noise, respectively for the two RX antennas.

The total energy levels of the received symbols r_(k1) and r_(k2) at thek^(th) RU are:E _(t,kl) =E _(s) ×|h _(kl)|² +E _(i,kl) +E _(w,kl),where l=1, 2, E_(s)×|h_(kl)|², E_(i,kl) and E_(w,kl) represent theaverage energy of the received symbols and the average energy of theinterference and noise received via the l^(th) receive antenna of thek^(th)RU, respectively. Generally, the average energies of the receivedsymbols, E_(s)|h_(kl)|², are different on different uplink channelsbecause the required SINR at these channels changes based on the PUCCHFormat (Format 1, 1a, 1b, 2, 2a, 2b) and the PUSCH MCS (e.g., QPSK or64-QAM). The interference energy, which is caused by other UEtransmissions in nearby cells, can also vary among different PRBs, whichcan cause additional variations in the energy levels of the receivedsymbols at the RUs.

The RUs implement the uplink compression using a quantizer to reduce thedata rate of transmissions from the RUs to the CUs. For the purpose ofdiscussion, we assume that the quantizer is a uniform scalar quantizerhaving a rate R_(kl) and a step size Δ_(kl) and quantizes the real andimaginary components of the received frequency-domain IQ symbolsindependently at the lth antenna of the k^(th) RU. Other quantizationtechniques, such as non-uniform scalar quantization or vectorquantization, can also be used with the techniques described herein.

Referring to FIG. 9, the RU 1300 sends the bits 1302 that represent anoutput of the quantizer 1304 to the CU 1306 in Ethernet frames throughan Ethernet network 1308. The CU 1306 reconstructs a quantized versionr_(kl)′ of each received symbol rid:r _(kl) ′=s×h _(kl) +i _(kl) +w _(kl) +q _(kl),where q_(kl) is the complex-valued quantization noise having an averageenergy E_(q,kl). The performance of the quantizer 1304 can be measuredby its signal-to-quantization noise ratio (SQNR), which is defined as:SQNR_(kl) =E _(t,kl) /E _(q,kl),where E_(q,kl)=2×MSE and MSE is the mean-squared error of the uniformscalar quantizer.

The quantized symbols are sent to the CU through the Ethernet network.In some implementations, the rate R_(k1) of the quantizer is chosen sothat the quantization noise does not substantially affect theperformance of the receivers at the CU. In the absence of quantizationnoise and assuming that the noise and interference received on all theantennas are uncorrelated, the performance of a receiver at the CU forMRC (a maximum-ratio combiner) can be represented by the effective SINR:SINR=Σ_(k)(SINR_(k1)+SINR_(k2)),where SINR_(kl)=E_(s)×|h_(kl)|²/(E_(i,kl)+E_(w,kl)) is the SINR on thelth RX antenna of the k^(th) RU.

When the interference i_(kl) on different RX antennas is correlated, theCU receiving the compressed symbols from the RUs can apply IRC(interference rejection combining). The performance of the IRC isdetermined based on the sum of the SINRs on all RX antennas as shown bythe above equation, except that each SINR for a given RX antennaincludes the effect of the spatial whitening filter.

Next, the effect of non-zero quantization noise on the performance ofthe receivers at the CU is considered. Thesignal-to-interference-plus-noise-plus-quantization noise-ratio at theoutput of the MRC receiver in the CU, SINR′, is:

${{SINR}^{\prime} = {\sum\limits_{k}\left( {{SINR}_{k\; 1}^{\prime} + {SINR}_{k\; 2}^{\prime}} \right)}},{where}$$\begin{matrix}{{SINR}_{k\; 1}^{\prime} = {E_{s} \times {{h_{k\; 1}}^{2}/\left( {E_{i,{k\; 1}} + E_{w,{k\; 1}} + E_{q,{k\; 1}}} \right)}}} \\{= {{SINR}_{k\; 1}/{\left( {1 + {\left( {1 + {SINR}_{k\; 1}} \right)/{SQNR}_{k\; 1}}} \right).}}}\end{matrix}$

In other words, the SINR′ is the sum of thesignal-to-interference-plus-noise-plus-quantization noise ratios at eachbranch of the MRC that receives quantized symbols from respectiveantennas in the cell. If the quantizer rates R_(kl) are chosen for allantennas (for all k and l) such that:SQNR_(kl)>>1+SINR_(kl),then SINR_(kl)′≈SINR_(kl), and SINR′ approximately equals the ideal SINRwith no quantization noise, e.g., SINR′≈SINR=Σ_(k)(SINR_(k1)+SINR_(k2)).

The amount of degradation caused by the non-zero quantization noise inthe effective SINR_(kl) for each antenna of the RU can also bedetermined using the above formula. The amount can be calculated asSINR_(kl)/SINR_(kl)′, which can be written as a function ofSQNR_(kl)/(1+SINR_(kl)).

Table 1 shows example amounts of degradation in SINR_(kl) per RX antennadue to quantization noise as a function of the ratioSQNR_(kl)/(1+SINR_(kl)). The data in this example illustrates that whenthe SQNR_(kl) is at least 20 dB above 1+SINR_(kl), the reduction inSINR_(kl) due to the quantization noise is less than 0.05 dB.

TABLE 1 Reduction in SINR_(kl) due to Quantization Noise. SQNR/(1 +SINR) (dB) SINR/SINR′ (dB) 0 3.01 5 1.19 10 0.41 15 0.14 20 0.04 25 0.01Quantization Methods

Below, four different example quantization methods for uplinkcompression are described, with an increasing compression ratio fromMethod I to Method IV.

Fixed Quantization

In this example method, a fixed uniform scalar quantizer having a fixedrate R_(kl)=R₀ and a fixed step size Δ_(kl)=Δ₀ is used. As an example,R₀=12 and the quantized IQ stream is sent from a RU to the CU at a totalbit rate of about 403 Mb/s for two RX antennas of the RU. Accordingly,the fixed quantizer having a step size of 12 bits can be implementedwithout a high level of complication and without substantially affectingthe performance of the signal transmission. The data rate of 403 Mb/sbetween the CU and the RUs is relatively high. When K RUs are sendingquantized frequency-domain IQ symbols at a data rate of 403 Mbps towardsthe CU for the same OFDM symbol, the aggregate bit rate between thenearest Ethernet switch and the CU is K×403 Mb/s, which can berelatively high for large K.

Adaptive Step-Size Quantization

In this example method, the quantization is implemented using a uniformscalar quantizer that has a fixed rate R_(kl)=R₀, and a step size Δ_(kl)that is adjusted dynamically. In some implementations, the step size maybe updated on a per-PRB basis and independently for each antenna. Foreach OFDM symbol, the step sizes Δ_(kl) are individually varied for eachuplink channel that uses resources on that OFDM symbol. For example,Δ_(kl) can be selected based on the average energy of thefrequency-domain IQ symbols received in each uplink channel. In someimplementations, the average energy of the IQ symbols on a given channelis estimated using the symbols to be quantized at the RUs. The step sizeof the quantizer can then be adjusted based on an assumed distributionof those symbols to be quantized. In some implementations, thedistribution is determined based on the size of the DFT used by the UE.In some implementations, optimizing the step size dynamically andindependently for each channel can allow signals to be transmitted fromthe RUs to the CU at a higher SQNR at the same data rate. In addition,optimizing the step size dynamically and independently for each channelcan be used to lower the data rate without reducing the SQNR.

In some implementations, it may not be necessary to vary the quantizerstep size Δ_(kl) in every OFDM symbol, e.g., when the average energy ofa symbol received by the RU from a UE does not vary significantly withina subframe. In such implementations, the step size for the first OFDMsymbol is determined using the received IQ symbols in the first OFDMsymbol, e.g., to avoid delay. When the number of symbols available isinsufficient to accurately estimate the average energy in the first OFDMsymbol, the average energy estimate and the step size can be refined insubsequent OFDM symbols.

The quantizer rate R₀ is chosen to be high enough so that theperformance of the receiver at the CU does not degrade for the highestMCS. For example, when R₀=10, the SQNR of the quantizer is about 52 dB(assuming a Gaussian input), which is more than 20 dB higher than theminimum SINR required for reliable communications at the highest uplinkMCS.

As shown in Method I, an SQNR that is 20 dB above the minimum requiredSINR allows the receiver at the CU to operate with a performancedegradation due to quantization of no more than 0.05 dB. A quantizerrate R₀ of 10 can produce an IQ data rate of about 336 Mb/s for two RXantennas of a RU. This represents a compression ratio of 10/12, or is17% higher compared to the compression rate of Method I. Because thequantizer rate R₀ is fixed, all frequency-domain IQ symbols received bythe RUs, including IQ symbols that carry no information, are quantizedand sent to the CU. When an optimized step size is used, the value ofthe quantizer rate required to achieve a desired SQNR is lower than whenthe step size is not optimized.

The RUs use different step sizes for different PUSCH/PUCCH/SRS/PRACHchannels based on information about the uplink channel boundariesreceived from the CU. In some implementations, the uplink channelboundaries for each PRB are indicated by downlink side information sentby the CU to the RUs. Referring to FIG. 10, the side information 1404for use in an uplink (UL) subframe N is sent by the CU 1400 in thedownlink (DL) subframe N−4 (1406) to the RUs 1402.

Examples of the downlink side information 1404, e.g., the contents andsizes, are as follows. The PUSCH or PUCCH PRBs assigned to the samechannel are consecutive, and the channel boundaries for PUSCH and PUCCHcan be indicated by a 6-bit position index and a 6-bit length field. TheCU can also send indications of the channel type (e.g., PUSCH, PUCCH,SRS, PRACH, etc.) to the RUs using a 2-bit index to facilitate the RUsto model the statistical distribution of the received symbols.Furthermore, one bit of the side information can be used to indicate thepresence of the SRS (sounding reference signal), which can occupy thelast OFDM symbol of the subframe. Also, the position of the PRACH, whenpresent, can be indicated by a 6-bit index.

Based on the knowledge of the PUSCH/PUCCH channel boundaries, the RUsdetermine for each OFDM symbol the average energy of the receivedfrequency-domain IQ symbols that belong to the same channel (or from thesame UE). The RUs then choose the step size Δ_(kl) of the quantizerbased on the determined average energy. In some implementations, a RUdetermines the optimum step size without distinguishing the differentchannel types (e.g., PUSCH or PUCCH). In some implementations, a RU usesthe downlink side information about the channel type to facilitatechoosing the optimum step size without any measurement related to thereceived frequency-domain IQ symbols (e.g., average energy). For theSRS, the RUs can estimate the average energy across the entire OFDMsymbol and determine the optimum step size. Alternatively, the RUs cansplit the subcarriers in an OFDM symbol that carries SRS into subbandsand optimize the step size for each subband. In some implementations, afixed pre-determined step size may be used to quantize the SRS signal.For the PRACH, the step size can be determined based on the peak powervalue of the received signal, or it may be fixed.

The RU may implement the uniform scalar quantization with variable stepsizes by applying a gain γ_(kl) to normalize the energy of the IQsymbols to be quantized. The RUs then quantize the real and imaginarycomponents of the IQ symbols using a uniform scalar quantizer having afixed step size Δ_(kl)=Δ_(l). In some implementations, the real andimaginary components are symmetric, and the same gain and scalarquantizer can be used for both the real and the imaginary components.

The RUs send uplink side information about the selected step sizes tothe CU, along with the data bits representing the quantizedfrequency-domain IQ symbols, based on which the CU reconstructs thereceived IQ symbols.

In some implementations, each step size of the quantizer is representedby a 12-bit index in the side information. In some implementations, theRUs update the step size in every OFDM symbol, which can increase theamount of side information transmitted in one time slot by up to afactor of 7. For the SRS, the RUs send to the CU the uplink sideinformation about the step size for each subband before sending thedata. For the PRACH, the information about the step size can be sentbefore the quantized PRACH data is sent.

Adaptive Rate and Step Size Quantization

In this method, in addition to dynamically adjusting the step sizeΔ_(kl) of the quantizer, the rate R_(kl) of the quantizer is alsodynamically adjusted for compressing (or quantizing) the IQ stream. Inan example, PUSCH symbols that carry user data and PUSCH symbols thatcarry UCI (uplink control information) are not distinguished. Also, thesame quantizer rate is applied to all symbols sent by the same UE.

The quantizer rate can be dynamically adjusted, for example on a per PRBbasis. As discussed previously, for PRBs that carry PUSCH IQ symbolsfrom a relatively low MCS, a lower quantizer rate can be used than therate for the PRBs carrying PUSCH IQ symbols from a relatively high MCS.Similarly, some PRBs carrying PUCCH symbols can be quantized at arelatively low rate. The SINR required for these PRBs (for PUSCH orPUCCH) to provide a reliable reception at the CU can be relatively low.Accordingly, these PRBs can tolerate a relatively high level ofquantization noise. Furthermore, those PRBs not carrying any data do notneed to be quantized. The high tolerance of quantization noise and thereduced number of PRBs to be quantized on the uplink can savetransmission bandwidth between the RUs and the CU. Adjusting thequantizer rate based on these considerations can reduce the average datarate on the uplink.

As discussed previously, the quantizer rate for each antenna of the RUis chosen to be relatively high such that SQNR_(kl)>>1+SINR_(kl), whereSQNR_(kl) is the quantizer SQNR and SINR_(kl) is the receiver SINR forthe l^(th) antenna of the k^(th)RU of a cell. When such a relationshipbetween the SQNR_(kl) and the SINR_(kl) is satisfied, the quantizationnoise is much lower than the interference plus noise seen on the antenna(l^(th) antenna of the k^(th) RU), and the effect of the quantization onSINR_(kl) is small.

In some implementations, a RU does not determine the SINR_(kl) on itsown. Instead, the RU learns from the CU the target SINR, SINR_(target),across all antennas of the cell. The SINR_(target) is a function of theMCS used in each PRB. The CU uses the power control loop to drive thetransmit powers of a UE to a baseline level, and the UE adjusts thebaseline transmit power according to the MCS used in a given PRB so thatthe SINR in the eNodeB is approximately equal to the SINR_(target).

In some implementations, the RUs choose the quantizer rate such that thequantization noise does not substantially reduce the SINR at thereceiver of the CU to below the target SINR. When the CU controls thetransmission power of the UE by accurately tracking channel changes, theSINR at the receiver of the CU approximately equals SINR_(target).Furthermore, when SQNR_(kl)>>SINR_(target)>SINR_(kl),SINR′=Σ_(k)(SINR_(k1)′+SINR_(k2)′)≈SINR_(target). In other words, insome implementations, the quantization noise does not substantiallyreduce the SINR at the receiver of the CU when the quantizer rate ischosen such that SQNR>>SINR_(target).

In summary, in some implementations, by selecting the quantizer ratesuch that SQNR_(kl)>>SINR_(target), a RU can quantize the IQ symbolswithout producing quantization noise that substantially affects theperformance of the CU receiver or prevents reliable communicationbetween the CU and the RU.

In the example techniques describe above, for a given PRB, the RUs inthe same cell use the same quantizer rate for all antennas. In someimplementations, the SINRs of different antennas (SINR_(kl)) can besignificantly different. In such implementations, different quantizerrates can be chosen for antennas having different SINR_(kl) in the samecell. For example, the quantizer rates can be chosen so that SQNR isproportional to 1+SINR_(kl). In particular, the quantizer rate for theantenna with a lower SINR_(kl) may be chosen to be lower than thequantizer rate for an antenna with a higher SINR_(kl). In someimplementations, when the SINR_(kl) of some antennas is too low relativeto the total SINR, it is wasteful for the RUs to which those antennasbelong to transmit the received IQ symbols to the CU. Significant IQstream compression can be achieved when those RUs can determine that thesignals received on their antennas do not contribute significantly tothe overall SINR in the CU and purge or prune the signals (which isequivalent to using a quantizer rate of “0” for these signals).

An RU can adjust the quantizer rate based on the SINR_(kl) seen on eachantenna and additionally, the difference between the SINR_(kl) on itsdifferent antennas and the SINR_(kl) on other antennas in the same cell.In some implementations, the CU selects RUs from which to receivesymbols. The CU can also determine the quantizer rate for each RU basedon past UE transmissions. For example, the CU sets the quantizer rate tobe zero for an antenna when it determines that the SINR_(kl) of thatantenna contributes to less than 5% of the total SINR.

In some implementations, the CU determines the quantizer rate for eachantenna on a per UE basis at the time when the UE transmits a PRACHpreamble. All RUs can be required to forward all PRACH preamble signalsto the CU so that the CU can make an initial determination of theSINR_(kl) for each antenna. The CU can then select the quantizer ratefor each antenna and include this information in the downlink sideinformation it sends to the RUs. The CU is capable of determining thequantizer rate for those RUs from which the CU receives PUSCH or PUCCHsignals transmitted by a UE in a recent subframe. For RUs whosetransmissions for a UE are purged, the CU can determine a quantizer ratebased on the SRS sent by the UE at regular intervals. All RUs can berequired to relay the SRS.

Based on the SRS and the PRACH preamble signals, the CU can determinethe quantizer rate for all RUs in a cell. In addition, the CU canperiodically request the RUs that previously have purged transmissionsfrom the UE to send IQ symbols and use the IQ symbols to update thequantizer rate for those RUs. By adjusting the quantizer rate fordifferent antennas, the average rate of the data sent from the RUs tothe CU can be significantly reduced, especially when there are many RUsin a cell.

In some implementations, purging signals on the PUCCH may be difficultwhen multiple UEs share the same PUCCH resources. In suchimplementations, symbols on the PUCCH are transmitted without purging.The uplink transmission rate is not substantially affected because thePUCCH occupies a variable but relatively small percentage of the uplinkresources. In some implementations, a fixed quantizer rate can be usedfor all antennas on the PRBs assigned to the PUCCH, even when PUCCHtransmissions implement transmit diversity in which the same controlinformation can be sent using different resources. In someimplementations, other, e.g., more sophisticated, quantization andpurging schemes can be used for the PUCCH when the radio network has avery large number (e.g., 16 or lager) of RUs in the cell.

The CU incorporates the quantizer rate for each PRB determined for eachantenna in the downlink side information, which is used by the RUs. Forthose unallocated PRBs that carry no data or for antennas that do notsignificantly contribute to total SINR, the CU sets the quantizer rateto be zero. The side information sent by the CU to the RUs can alsoinclude other information, such as PUSCH MCS and PUCCH Format, and anindex that represents the expected probability distribution of thefrequency-domain IQ symbols in the PRB.

Similar to Method II, the CU sends the side information associated withuplink subframe N in downlink subframe N−4 (see, e.g., FIG. 10). The RUsuse the side information received in downlink subframe N−4 to select thequantizer step size for each PRB in uplink subframe N. The step sizesare optimized similar to Method II, e.g., based on the measured averageenergy and the estimated probability distribution of the received IQsymbols. The RUs send the selected step size for each quantizer to theCU at the beginning of each OFDM symbol before transmitting thequantized IQ symbols. Generally, little uplink capacity is used to sendthe side information for the step sizes.

Quantization based on Method III may not reduce the peak rate of theuplink IQ data compared to Method II. However, the method cansignificantly lower the average bit rate. As an example, the average bitrate can be reduced by more than 50%, when only 50% of the uplinkresources are in use. Adapting the quantizer rate using the techniquesof this method can help reduce the average uplink data rate and the loadon the Ethernet switches.

Predictive Quantization

In the previously example described Methods I, II, and III, the signalsreceived on different antennas of the same RU are treated asuncorrelated. In this fourth example method, when the number of receiveantennas is greater than the number of layers sent by a UE in spatialmultiplexing, the correlation between signals received on differentantennas of the same RU is used to further reduce the quantizer rate forPUSCH transmissions. In the Release 10 version of the LTE standard, theUE may transmit on multiple antenna ports. For the purpose ofdiscussion, it is assumed that the UE transmits on the PUSCH using asingle transmit antenna port.

As shown previously, signals received by the two antennas of the k^(th)RU in a cell can be represented as:r _(k1) =s×h _(k1) +i _(k1) +w _(k1),r _(k2) =s×h _(k2) +i _(k2) +w _(k2).Furthermore, r_(k2) can be expressed according to the followingpredictor equation:r _(k2) =a _(k2) ×r _(k1) +z _(k2),where the prediction coefficient a_(k2) is given by:a _(k2) =E{r _(k2) r _(k1) *}/E{|r _(k1)|²},and z_(k2) is the prediction error and can be written as:z _(k2) =r _(k2) −a _(k2) r _(k1).

An RU can estimate the prediction coefficient a_(k2) by calculating theaverage correlation between the signals received at the two antennas,and then dividing the result by the average energy of the signalsreceived on the second antenna. The RU performs the estimation on a perUE basis based on information received from the CU.

Referring to FIG. 11, the RU first quantizes r_(k1) with a uniformscalar quantizer having a rate R_(k1) and a step size Δ_(k1) to obtainthe first quantized signal r_(k1)′ 1502, wherer _(k1) ′=r _(k1) +q _(k1).Here q_(k1) is the quantization noise for the symbol received at thefirst antenna. The RU then uses r_(k1)′ to produce 1504 the predictionerror z_(k2)′=r_(k2)−a_(k2)r_(k1)′, which is then quantized with anotheruniform scalar quantizer 1506 having a rate R_(k2) and a step sizeΔ_(k2) to generate the second quantized signal.z _(k2) ″=r _(k2) −a _(k2) r _(k1) ′+q _(k2).Here q_(k2) is the quantization noise for the symbol received at thesecond antenna. Bits 1510, 1512 representing the quantized symbolsr_(k1)′ and z_(k2)″ are sent to the CU, along with the predictioncoefficient a_(k2) and the quantizer information R_(k1), R_(k2), Δ_(k1)and Δ_(k2). The CU first reconstructs 1514, 1516 the quantized symbolsr_(k1)′ and z_(k2)′ and then generates the quantized symbol r_(k2)′ 1518according tor _(k2) ′=z _(k2) ″+a _(k2) ×r _(k1) ′=r _(k2) +q _(k2).

The average energy of the symbol z_(k2)″ is lower than that of r_(k2),and the quantizer rate R_(k2) is generally lower than the quantizer rateused when the RU quantizes r_(k2) directly without prediction. The lowerquantizer rate can reduce the IQ rate.

Again, the SINR in the CU can be written as:SINR′=Σ_(k)(SINR_(k1)′+SINR_(k2)′),where SINR_(kl)′=E_(s)×|h_(kl)|²/(E_(i,kl)+E_(w,kl)+E_(q,kl)).

For the first antenna, SINR_(k1)′ can be written as:SINR_(k1)′=SINR_(k1)/(1+(1+SINR_(k1))/SQNR_(k1)).Accordingly, in this example, when the quantizer rate for the firstantenna is chosen such that SQNR_(k1)>>1+SINR_(k1), the quantizationnoise does not substantially affect SINR_(k1)′.

Similarly, for the second antenna, SINR_(k2)′ can be written as:SINR_(k2)′=SINR_(k2)/(1+[(1+SINR_(k))/(1+SINR_(k1))/SQNR_(k2)])).Here SINR_(k)=SINR_(k1)′+SINR_(k2)′ and is the total SINR in the CU forthe k^(th) RU. Accordingly, in this example, whenSQNR_(k2)>>(1+SINR_(k))/(1+SINR_(k1)), the quantization noise introducedby the second quantizer does not substantially affect SINR_(k2)′.

In some implementations, the two antennas of a RU have the same SINR,e.g., SINR_(k1)=SINR_(k2), and the condition for the quantization noiseto not substantially affect the SINR at the CU can be simplified to:SQNR_(k2)>>(1+SINR_(k))/(1+0.5×SINR_(k)).When SINR_(k)>>1, SQNR_(k2)>>2. A uniform scalar quantizer having a rateof about 5-6 can readily satisfy this condition. The resulting IQ ratefor the 2^(nd) antenna is reduced to about 84-101 Mb/s, representing acompression of more than 50%.

To implement the predictive quantization, in some implementations, theCU estimates the prediction coefficients, in addition to determining thequantization rate based on the predictive quantization. The estimatedcoefficients can be sent to the RUs in the downlink side information.Alternatively, the CU can determine the quantizer rate as discussed inMethod III and without relying on predictive quantization. The RUs applythe prediction and send the prediction coefficient as part of the uplinkside information to the CU. In some implementations, the CU determinesthe quantizer rate based on the predictive quantization, and the RUsdetermine the prediction coefficients and send the coefficients to theCU as part of the uplink side information.

Uplink Compression of the PRACH Preamble

When an idling UE has data to send or to receive, the UE establishes aconnection with the eNodeB by sending a PRACH preamble to the eNodeB insome designated PRBs that are shared by all the UEs in a cell. In someimplementations, each cell has 64 shared PRACH preamble sequences, someof which are designated for use in contention-free access and the othersare divided into two subsets. In contention-free access, the eNodeBassigns a preamble to the UE. In other situations, the UE selects one ofthe two subsets based on the amount of data to be transmitted. The UEthen randomly picks one of the preamble sequences in the selectedsubset.

A PRACH preamble uses 6 RBs at 1.08 MHz, and the positions of the 6 RBsare determined and signaled to the UEs by the CU. The PRACH preamble canlast 1, 2 or 3 subframes, depending on the length of the cyclic prefixand the guard time. The PRACH opportunities can occur as frequently asonce every 1 ms subframe or as infrequently as once every 20 ms.

In general, the UEs are not scheduled to transmit PUSCH on the PRBsassigned to PRACH. The CU can use non-adaptive HARQ on the uplink toprevent collisions between PRACH and HARQ retransmissions. Thenon-adaptive HARQ changes the RBs used in the transmission for collisionavoidance. The PRACH opportunities can also be chosen to not overlapwith the SRS or the PUCCH transmissions. The UE selects the transmitpower for the PRACH preamble based on open-loop power control where theUE estimates the uplink signal loss based on a measurement of thedownlink signal loss and gradually increases the transmit power afterunsuccessful attempts.

The detection of the PRACH preamble can be implemented partially in theRU and partially in the CU. In some implementations, the RUs know theexact position of the PRACH opportunities and convert the receivedtime-domain IQ symbols (at 15.36 MHz for the 10 MHz FDD LTE standards)into a lower-rate time-domain sequence (e.g., a rate of 1.28 MHz) usinga time-domain frequency shift followed by decimation. The resultingsequence is then converted to frequency domain using an FFT (e.g., a1024-point FFT for the 10 MHz FDD LTE standards). A frequency-domaincorrelation is performed between the FFT output and the frequency-domainrepresentation of the root Zadoff-Chu sequence. The 64 PRACH preamblesequences are derived using a cyclic shift. The complex-valued output ofthe frequency-domain correlator is then converted back to acomplex-valued time domain sequence using an IFFT (e.g., a 1024-pointIFFT).

The RUs and the CU perform the next steps of detecting the PRACHcollaboratively. For example, the RUs can compute a real-valuedtime-domain sequence of 1024 samples by summing the squares of the realand the imaginary components. The RUs can send this information to theCU for further processing. The CU, upon receiving the time-domain powersequence, performs a peak detection to determine the preamble cyclicshift. Such uplink PRACH transmissions are compressed in the time-domainsuch that data compressed in the time-domain is transmitted between theRUs and the CU.

Alternatively, the RUs can send the complex-valued output symbols of theIFFT to the CU and let the CU perform the remainder of the PRACHpreamble detection. In some implementations, the RUs implement the peakdetection, determine the preamble cyclic shift, and send the CU thecyclic shift information. The amount of data transmitted from the RUs tothe CU for PRACH preamble detection is small. In the example of the 10MHz FDD LTE, the amount of data ranges from a few bits to 12-20 Mb/s,depending on whether the real-valued power or the complex-valued IFFToutputs are sent.

In some implementations, when there is no substantial overlap betweenthe PRACH transmissions and other uplink transmissions, no othertransmissions are performed for the RBs that are transmitted on thePRACH.

For the RUs to correctly implement the PRACH preamble detection, the CUcan provide the RUs with configuration information, such as the PRACHconfiguration index, PRACH frequency offset, PRACH Zadoff-Chu rootsequence, etc. The CU can send this information to the RUs when the RUsare initially assigned to the CU or when the PRACH is modified.

The PRACH data may be quantized with a fixed rate quantizer, whose rateis pre-determined by the CU and sent to the RUs when the RUs initiallyconnect to the CU. The quantizer step size may also be fixed, or it maybe dynamically selected by the RUs based on the average energy of thereceived PRACH signal.

Synchronization

With the example systems described herein, there may be synchronizationrequirements that may not be applicable to classic base stations.

As explained above, in some example systems described herein, some partsof the baseband processing (e.g., modem functionality) and FFT/RFprocessing (e.g., radio functionality) are split between a central CUand multiple RUs (RUs) that are connected via a switched Ethernetnetwork (as shown in the figures). In classic base stations, a GPSreceiver is typically used to acquire time and frequency synchronizationand since the modem and RF functions are co-located, they can besynchronized to the GPS receiver. In the example systems describedherein, in some implementations, a GPS receiver is only available in theCU, and is not available in the RUs to keep the system cost low and toavoid the installation complexity. The CU can also acquire timing andfrequency synchronization in other ways, for example from a networkserver or by listening to signals transmitted by a macro cell basestation nearby. In some implementations, a timing transport protocol isused to carry a stable absolute timing phase and frequency referencethat is traceable to coordinated universal time (UTC/GPS) from the CU tothe RUs. The timing transport protocol can be based on the IEEE1588protocol. In some implementations, clock frequency and the absolutetiming phase derived by the RUs should be accurate enough to meet all3GPP synchronization requirements and to ensure that UEs performance isnot noticeably impacted by any frequency or timing phase error betweenthe RUs and the CU and between the RUs themselves.

To deal with the variable packet delays in an Ethernet network, downlinkair interface framing in the CU and uplink air interface framing in theRUs may be advanced by T_(DL) and T_(UL) seconds relative to each other.In some implementations, these framing advances T_(DL) and T_(UL) haveto be greater than a sum of the respective Ethernet network delaybetween the CU and the RU and the timing phase error between the clocksin the CU and the RU. Since the worst-case clock error is small comparedto the worst-case Ethernet delay, it has a lesser effect on theselection of the framing advances T_(DL) and T_(UL). When the actualnetwork delay that a packet experiences exceeds the framing advance,buffer underflow will occur and physical layer transport packets will belost. Such a loss can be recovered using retransmissions in HARQ, RLP orTCP layers, but at the expense of reduced transmission efficiency.Therefore, in some implementations, it is important that such underflowoccurs rarely, and does not impact the user experience.

One of the features of the example systems described herein is theirability to serve UEs via multiple RUs that share the same cell. Forexample, as described above, multiple RUs may be controlled by a CU todefine a cell, in which multiple UEs may be served. Assigning multipleRUs to the same cell may reduce the number of baseband modems used inthe CU, avoid inter-cell interference and improve signal strengththrough macro-diversity. Sharing the same cell across multiple RUs mayreduce the LTE system capacity available to individual users. In someimplementations, as long as cell loading remains below 50% of cellcapacity, no appreciable performance degradation will occur.

In order to implement cell sharing in the example systems describedherein, in some implementations, the relative carrier frequencies of RUsin the same cell should be frequency synchronized in a way that istighter than the frequency accuracy required from individual RUs. Insome implementations, without such tight differential synchronization,the effective downlink channel seen by the UE may become time-varying ina manner similar to what happens when there is mobility and as a resultthe performance may degrade. Channel variations caused by mobility or bydifferential carrier frequency offset between RUs result in a mismatchbetween the channel measured using the reference signals and the channelactually experienced when demodulating the LTE OrthogonalFrequency-Division Multiplexing (OFDM) symbol.

The tight differential carrier frequency synchronization of RUs asdescribed above may also be required between RUs that belong todifferent cells but use Rel. 11 downlink Coordinated Multipoint (Rel. 11CoMP or simply “CoMP”). In CoMP, at a cell-edge, typically, downlinksignals from two or more RUs that may belong to different cells could bereceived at a UE while the UE's uplink transmissions could also bereceived by these various RUs. If the downlink transmissions to a givenUE can be coordinated, downlink performance can be enhanced. Likewise,if uplink transmissions can be scheduled in a coordinated manner, uplinkperformance can be enhanced. CoMP addresses issues such as interferencemitigation and coordinated bit transmissions over the air interface.

When such tight synchronization cannot be maintained, downlink physicallayer CoMP performance may degrade, potential CoMP gains may be reducedor lost or could even turn negative. Downlink CoMP is a part of thepresent disclosure, but tight differential synchronization requirementsfor some implementations of CoMP are not unique to the presentdisclosure and also apply to other LTE systems that use downlink CoMP.

When multiple RUs share the same cell, the timing phase of theirtransmissions also needs to be synchronized. This synchronization canalso facilitate the radio network of this disclosure to combine uplinksignals received by different RUs in the CU. In some implementations,such combinations require that all significant multipath signalsreceived by different antennas fall within a time interval called cyclicprefix. The cyclic prefix corresponds to the first N_(CP) samples in anOFDM symbol that are a replica of the last N_(CP) samples in the samesymbol. The cyclic prefix ensures that the transmitted subcarrier willremain orthogonal in the receiver, as long as the delay spread of thechannel is less than the N_(CP). When multiple RUs share the same celland there is a timing phase offset between the RUs, the sum of thisoffset and the delay spread of the wireless channel can be controlled soas to not exceed the cyclic prefix length. In the LTE standard, thecyclic prefix is around 5 milliseconds. Therefore, it is desirable tokeep the timing phase error between RUs much smaller than 5milliseconds.

Following an explanation of example synchronization requirements forsome implementations, there is also a description of how theserequirements are addressed.

In this regard, synchronization, and the features described hereinrelating thereto, are example implementations. Different implementationsof the example systems described herein may employ differentsynchronization methods and variations on any and all of the methodsdescribed herein. Any requirements specified in this disclosure relateto the specific example implementations described herein only, and arenot requirements of any more general methods, apparatus, systems, andcomputer program products that may be claimed.

In an example implementation of the present disclosure, basebandoperations up to the FFT input are performed in the CU and the remainingbaseband operations (FFT, cyclic prefix, etc.) and the radios areimplemented in the RUs. In another example implementation, on thedownlink, baseband operations up to the modulation or layer mapping areimplemented in the controller and the remaining baseband operations areimplemented in the RUs. As previously explained, the CU and the RUs areseparated by a switched Ethernet network that carries data between theCU and the RUs in packets or frames.

Synchronization Between the CU and the RUs

In some implementations, there is a VCTCXO crystal oscillator in the CUand VCTCXO crystal oscillators in all of the RUs. The VCTCXO in the CUis used to generate clocks required for the baseband processing in theCU and the VCTCXOs in the RUs are used to generate clocks foranalog-digital-analog converters (A/D/As), RF synthesizers, and basebandprocessing performed in the RUs. In some implementations, only the CUhas a GPS receiver or another timing synchronization mechanism that cangenerate a stable frequency-stable and phase-accurate clock referenceand, therefore, there is a need to provide a frequency-stable andphase-accurate clock reference to the VCTCXOs in the RUs using IEEE1588based timing synchronization. As described by the National Institute ofStandards and Technology (NIST), the IEEE 1588 standard “defines aprotocol enabling precise synchronization of clocks in measurement andcontrol systems implemented with technologies such as networkcommunication, local computing and distributed objects. The protocol . .. [is] . . . applicable to systems communicating by local area networkssupporting multicast messaging including but not limited to Ethernet”.The contents of the IEEE 1588-2002 as published in 2002 and as revisedin 2008 are hereby incorporated by reference into this disclosure.

IEEE1588 is a time-stamping protocol, implemented over the UDP/IPprotocol, between a master clock in the CU and slave clocks in the RU.The protocol involves repeated round-trip exchanges between the masterand slave clocks, where each exchange produces a timing update signalthat can be used to construct a timing reference signal in the RU. Themaster clock starts the exchange by sending a time stamp to the slave inthe RU. This time stamp carries the time T1 as measured by the masterclock at the time the time stamp leaves the Ethernet interface on theCU. The slave receives this time stamp when its local clock is at timeT1′. The difference T1′−T1=D_(DL)+Δ is the sum of the unknown one-waytravel delay D_(DL) of the time stamp from the CU to the RU and theunknown clock phase error Δ between the reference clock in the RU andthe reference clock in the CU. In order to estimate (and cancel) theone-way downlink delay, the slave sends to the CU a second time stamp.This time stamp carries the time T2 as measured by the slave clock atthe time the time stamp leaves the Ethernet interface on the RU. Themaster marks the time T2′ on its local clock when it receives the timestamp on the Ethernet interface on the CU, and sends value T2′ in aresponse message back to the slave. The difference T2′−T2=D_(UL)−Δ isthe sum of the unknown one-way travel delay of the time stamp from theRU to the CU and the unknown clock phase error (−Δ) between thereference clock in the CU and the reference clock in the RU. If theone-way delay in the two directions were the same (e.g., D_(DL)=D_(UL))and the phase of the reference clock in the CU does not drift relativeto the reference clock in the RU during the exchange, the slave canestimate the clock error Δ by removing the effect of the one-way delaysby computing:Δ′=[(T1′−T1)−(T2′−T2)]/2.This clock phase error estimate Δ′ can be used in the RU to produce areference signal that closely tracks the timing reference signal (e.g.,a GPS-derived, 1 Pulse Per Second (1PPS) signal) in the CU.

In some implementations, the one-way delays in the two directions aregenerally not equal, primarily due to asymmetric load-dependent delaysin the switches (propagation and transmission delays are typicallysymmetric). To reduce the effect of such errors, IEEE 1588v2 introducedthe ability for intermediate nodes, such as Ethernet switches, tomeasure the delays that the packets incur inside the node and insertthis part of the delay into the time stamp packets as they traverse thenode. Such 1588v2 support by Ethernet switches will allow the slave toestimate the round-trip delay without the asymmetric load-dependentnetwork delays and produce a much more accurate estimate of the clockoffset to drive the Phase Locked Loop (PLL). However, switches thatsupport IEEE1588 tend to be more expensive and therefore there is a needto develop methods that can reduce or eliminate the effects ofasymmetric network delays.

To the extent the IEEE1588v2 processes can be used to drive the timingphase error to zero, the reference clock in the RU can be perfectlyaligned in phase and frequency with the reference clock in the CU, forexample a GPS 1PPS signal.

In some implementations of the example systems described herein, theVCTCXO in the CU is used as the master clock to generate the timestampsfor the IEEE1588 protocol. The RU's VCTCXO is disciplined using the timestamps received by the IEEE1588 slave. Intelligent time stamptransmission and processing may be used in the CU and the RUs to reduceor eliminate jitter introduced by random asymmetric Ethernet networkdelays between the CU and the RU. The timing of timestamp generation inthe CU and in the RUs is orchestrated to reduce asymmetric delays.Timestamp generation and processing may be implemented on aSystem-on-Chip (SoC) in both the CU and the RU. Hardware-assist is usedin this process to reduce the possibility that random asymmetric delaysare introduced into the IEEE1588 processing.

If the time stamps are sent by the CUs and RUs in an uncoordinatedmanner, they may experience different delays on the uplink and downlinkbecause of different levels of contention they encounter in the twodirections. For example, if multiple RUs respond to a time stamp sent bythe CU at about the same time, the uplink time stamps may experiencesignificantly longer delays than the time stamps sent on the downlink.Contention between time stamps and IQ data may also contribute toincreased latency and such latency may be different in the twodirections.

Two metrics that can be used to assess the accuracy of the IEEE1588timing synchronization method are the mean value and the variance of theclock error estimate Δ′:

$\begin{matrix}{{\left. {{E\left\{ \Delta^{\prime} \right\}} = {{E\left\{ \left( {{T\; 1^{\prime}} - {T\; 1}} \right) \right\}} - {E\left\{ \left( {{T\; 2^{\prime}} - {T\; 2}} \right) \right\}}}} \right\rbrack/2} = \left\lbrack {{E\left\{ {D_{DL} + \Delta} \right\}} -} \right.} \\{{\left. {E\left\{ {D_{UL} - \Delta} \right\}} \right\rbrack/2} =} \\{{= {\Delta + {E{\left\{ {D_{DL} - D_{UL}} \right\}/2}}}},}\end{matrix}$where E{ } refers to statistical expectation or mean value of itsargument. In other words, the mean of the timing estimate Δ′ has a fixedbias which corresponds to the average delay difference between theuplink and the downlink, divided by 2. When the average delays on the DLand UL differ significantly, there could be a significant phase error inthe average timing estimate. The variance of the timing estimate isproportional to the variance of ½ the difference between DL and ULdelays.E{(Δ′−E{Δ′})²}=variance{(D _(DL) −D _(UL))/2}.

The mean-squared estimation error E{(Δ′−Δ)²} between the estimated clockphase error and the actual clock phase error will be higher than thevariance of Δ′ by the square of the bias:E{(Δ′−Δ)²}=variance{D _(DL) −D _(UL)/2}+[E{D _(DL) −D _(UL)}/2]².

In some implementations, it is possible for the RU to accuratelydetermine the ratio between the UL and DL delays; e.g., D_(UL)/D_(DL)=a.The RU can then modify the formula for the clock error estimateaccording to:Δ′=[a(T1′−T1)−(T2′−T2)]/(1+a).

To the extent the parameter “a” can be determined exactly, a perfectestimate of the clock error can be obtained with no bias; e.g., E{Δ′}=Δand variance{Δ′}=0. In some implementations, it is difficult to know theuplink and downlink delays exactly in a consistent manner. Sometimes itmay be possible to determine a functional relationship between theuplink and downlink delays on average. For example, if there is a knownfunctional relationship between the average delays D₁=E{D_(DL)} andD₂=E{D_(UL)}, then it is possible to reduce or even remove the bias termE{D_(DL)−D_(UL)}/2. For example, if D₂=a D₁+b, in other words theaverage delay in the UL is a known linear function of the average delayon the DL, then we can reduce or remove the bias by using a modifiedtiming estimate given by the following:Δ′=[a(T1′−T1)+b−(T2′−T2)]/(1+a).

In this case, it can be shown that E{Δ′}=Δ, which is the correctestimate with no bias. It can be observed that in the special case wherea=1 and b=0, this reduces to the case where the average delays on the ULand DL are the same and the timing estimate reduces to the standard 1588timing estimation formula.

The variance of the timing phase estimate is now reduced to:E{(Δ′−E{Δ′})² }=E{(Δ′−Δ′)²}=variance{a D _(DL) +b−D _(UL)/(1+a)}.

Another method for reducing the mean-squared timing phase error is toreduce (e.g., minimize) both the mean and the variance of the averagedelay differential between the uplink and the downlink by controllingthe transmission of the time stamps relative to each other and relativeto the IQ data transmissions between the CU and the RU so as to avoidcontention in the switches. Described below is an example method thatmay reduce the downlink and uplink delays.

In this method, the CU and each RU executes multiple time stampexchanges during a given time interval A, e.g., 1 second. For example,the CU and the RU may execute 10 time stamp exchanges during a 1 secondinterval, where each time stamp exchange uses 3 IEEE1588 messagetransmissions as described earlier. In some implementations, referringto FIG. 18, the CU sends 2502 its time stamp in the beginning of theOFDM symbol interval. The CU then waits 2504 for some pre-configuredperiod of time before transmitting 2506 its IQ data to allow time forthe time stamp to travel through the switches. The time stamptransmissions are associated with of the highest priority. If a timestamp encounters contention from IQ data in the switches, it will atmost wait for the transmission of the IQ data whose transmission hasalready started. Upon receiving 2508 the time stamp, the RU initiatesthe transmission of its own time stamp at randomly chosen intervalslater. In some implementations, upon receiving the time stamp from theCU, the RU may wait 2510 a pre-configured time interval beforetransmitting 2512 the time stamp. The pre-configured time interval mayalso depend on the time when the RUs own uplink IQ data transmission iscompleted. The CU, upon receiving 2514 the RU's time stamp, marks 2516the time on its local clock and sends this measured time to the RU inanother IEEE1588 message. The RU upon receiving 2520 this messagecalculates 2522 an estimate of the clock phase error (or equivalently, aclock offset), but does not make any adjustment to its clock. In someimplementations, the CU and the RU repeat the above exchange multipletimes during the time interval A. At the end of the time interval, theRU compares 2524 the clock offsets and updates 2524 its clock based onthe measurement that corresponds to the lowest clock offset.

In some implementations, the RU may compare the clock offset to athreshold value. If the clock offset exceeds the threshold value inmagnitude, the RU does not update its clock during an interval A. Inaddition to computing the estimates for the clock offset, the RU candetermine the round trip delay asD _(DL) +D _(UL)=[(T1′−T1)+(T2′−T2)].A round trip delay may indicate that the IEEE1588 exchange hascontention, and that that the associated clock offset is inaccurate, andtherefore, should not be used.

The CU also implements similar IEEE1588 exchanges with other RUs. Insome implementations the CU may implement the IEEE1588 exchanges withdifferent RUs in a non-overlapping fashion, so as to minimize contentionin uplink time stamp transmissions. In some implementations, only oneIEEE1588 exchange may be used for each RU during the time interval A.

If there are multiple controllers at the site sending traffic to thesame output port of a switch, these transmissions may also createcontention and increase latency. One way such contention may be avoidedis to use a single controller to act as the master for all DLtransmissions. In other words, all traffic may be routed through themaster controller. Alternatively, a single controller may assume themaster role only for the IEEE1588 operation. In this case, only themaster controller will send time stamps to the RUs.

If the RUs and the controller support other traffic, such as Wi-Fitraffic, the transmission of the other traffic may also be timed toavoid contention in the switches. For example, additional Ethernet linksmay be used to avoid direct contention between such other traffic andthe latency-sensitive IQ data and IEEE1588 time stamp traffic.

In some implementations, traffic associated with different controllersand other traffic, such as WiFi, can be segregated, e.g., strictlysegregated, by assigning them to different VLANs and using dedicatedEthernet links and ports for the radio network to avoid contention.Ethernet QoS capabilities can be implemented to improve the performanceof the above methods. Using priority levels defined in the 802.1pstandard, time stamp transmissions can be given higher priority tominimize delays in switches that may be caused by IQ data transmissions.

Next, a description is provided of how uplink and downlink subframestransmitted across the switched Ethernet network should be aligned.

Frame Advance

Aligning the downlink and uplink transmissions at the antennas in astandalone eNodeB can create a slight misalignment in the eNodeBbaseband processor. But, since the delay between the antennas and thebaseband processor is relatively small, this has little, if any, impacton the system performance. However, in some implementations, a delaybetween baseband processing in the CU and the antennas near the RUs canbe significantly higher than in a standalone eNodeB because of thedelays introduced by the Ethernet network between the CU and the RUs. Insome cases, the fixed delay between the CU and the RU can be in theorder of 200-300 μs, or 3-4 OFDM symbol intervals. To compensate forthis delay, one may advance the downlink subframe timing in the CU by apre-determined amount of T_(DL) seconds, where T_(DL) is on the order of200-300 μs in some implementations. If the uplink (UL) and downlink (DL)frames are aligned at the RU antenna then, as described below, an offsetwill occur between the UL and DL subframes in the baseband modem of theCU. One timing synchronization requirement in LTE is related to therelative timing phase of uplink transmissions from different UEs. Thisrequirement, called the Uplink Timing Advance, is also implemented inthe present disclosure. In Uplink Timing Advance, the UEs advance thetiming phase of their uplink transmissions relative to received downlinktransmissions based on commands received from the eNodeB. A standardeNodeB determines the timing advance commands to align the start of thereceived n'^(th) uplink subframe with the start of its own downlinktransmission of the n'^(th) subframe at the antennas. If the UE's timingadvance is set equal to the round-trip delay between the UE and theeNodeB antennas, the uplink signals from different UEs will bephase-aligned at the eNodeB antennas.

Accordingly, in the example systems described herein, uplink signalsfrom different UEs are timing-phase aligned at the receive antennas ofthe RU such that these transmissions are all received within the cyclicprefix as explained earlier. One can then choose the timing advance (TA)according to TA=t_(RT), where t_(RT) is the mean round-trip delaybetween the UE and the nearby RU antennas. This would automaticallyalign the DL subframe boundaries, which are phase-aligned with GPS 1PPS,with UL subframe boundaries at the RU antenna as shown in FIG. 12.However, the DL and UL subframe boundaries at the CU are now offset withrespect to each other by T_(RT)=T_(DL)+T_(UL), where T_(DL) and T_(UL)are the assumed fixed downlink and uplink frame timing advance betweenthe CU and the RU, respectively. In summary, in the RU TX (transmit)antenna, the transmission of the n'th DL subframe starts at the sametime as the reception of the n'th UL subframe, but in the CU thereception of the n'th UL frame occurs T_(RT) seconds later than thestart of the transmission of the n'th DL subframe. A drawback of thisapproach is that the HARQ processing time in the CU may be reduced byT_(RT) seconds, which can be as high as 500 μs. In implementations wherethere is no delay between the RU and the CU, the controller has 3 msavailable to process the signals received on the uplink and start thecorresponding transmission on the downlink. Therefore, this couldrepresent a reduction of 17% in processing time available in the CU.

Consider the downlink HARQ operation of FIG. 13, where the CU sendsPDSCH data in DL subframe N, which is received by the UE afterT_(DL)+t_(DL) seconds. The UE sends an ACK/NAK message in uplinksubframe N+4. If the timing advance TA=t_(RT), as would be the case in aclassic eNodeB, then from the end of DL subframe N to the beginning ofUL subframe N+4, the UE has 3−TA=3−t_(RT) ms to demodulate the DLsubframe N, determine the ACK/NAK and construct the ACK/NAK message.From the time it receives the UL subframe N+4 carrying the ACK/NAK, theCU can have until the beginning of DL subframe N+8 to schedule aretransmission. When TA=t_(RT), then from the end of the N+4'th ULsubframe to the beginning of the N+8'th DL subframe, the CU will haveonly 3−T_(RT) ms available to start a retransmission. In other words,the available processing time in the CU is reduced by the round-tripdelay between the CU and the antenna. In some implementations, the CUmay delay the retransmission by taking advantage of so-called adaptivefeature of the DL HARQ, though in some circumstances this may reduce theoverall throughput. A similar reduction in available processing timealso occurs in uplink HARQ, where the CU has 3−(T_(DL)+T_(UL))processing time between receiving an uplink transmission and sending anACK/NAK on the downlink.

A method that can address the above issue is to increase the uplinktiming advance TA by T_(RT) for all the UEs. In some implementations,this does not affect the uplink timing phase alignment among UEs at theRU, since the timing advance is increased by the same amount for all theUEs. As explained above, increasing the timing advance reduces the HARQprocessing time in the UE, but since all the UEs are designed to handlea maximum timing advance of 667 μs in some implementations, there shouldnot be any problems as long as the timing advance is kept below thislimit. The subframe alignment in this case is illustrated in FIG. 14.

As required, the DL subframes are phase aligned with GPS 1PPS at the TXantennas, but the UL subframes at the RX antennas are now offset byT_(RT) seconds relative to GPS 1PPS. In other words, the RU will startprocessing UL subframe N T_(RT) seconds before it starts processing DLsubframe N.

The revised HARQ timing for both downlink and the uplink are illustratedin FIGS. 15 and 16. In the examples shown in both figures, theprocessing time in the CU remains constant at 3 ms, whereas theprocessing time in the UE is reduced to 3−t_(RT)−T_(RT) ms, but is stillwithin the bounds of UE's capabilities. It is possible to choose thetiming advance to be anywhere between t_(RT) and t_(RT)+T_(RT).

Other enhancements also support excess timing advance to compensate forextra delay between the CU and the RUs. For example, the CU may send aLayer 2 or Layer 3 timing advance message to the UE to separately signalthe component of timing advance that relates to over-the-air delay andthe component that relates to delay between the CU and the RU. The fixedpart of the timing advance may be included in a cell-specific broadcastmessage, such as a SIB message in LTE.

It is also possible to make the excess timing advance UE-specific.

When the UE applies a large timing advance TA, the preambleconfiguration for the Physical Random Access Channel (PRACH) needs to beselected accordingly to prevent the PRACH preamble transmission insubframe N from interfering with Physical Uplink Shared Channel(PUSCH)/Physical Uplink Control CHannel (PUCCH) transmissions in thenext subframe N+1. The guard interval GI of the preamble should begreater than the timing advance TA or alternatively, the eNodeB shouldnot schedule any PUSCH transmissions in subframe N+1 in the resourceblocks assigned to PRACH in subframe N. PRACH preamble formats 1 and 3support a TA of up to 0.52 and 0.72 ms, but use 2 and 3 subframes,respectively. PRACH preamble formats 0 and 2 only support a TA of up to0.1 and 0.2 ms, using 1 and 2 subframes, respectively. PRACH preambleformat 1 is suitable for the present disclosure if TA can be kept below0.5 ms. Alternatively it is possible to use a format 0 and not toschedule any PUSCH transmission in the PRACH RBs in the uplink subframeimmediately following the PRACH transmission.

In the CU the PRACH packets sent by the RU are stored in a PRACH buffer,separate from the UL buffer, and are processed as quickly as possible.The CU examines the 839-point energy sequence, determines whether apreamble is present and, if so, estimates the cyclic shift that wastransmitted by the UE, and prepares the PRACH response message. Whilethere is no strict timing requirement for the CU to send a PRACHresponse, in some implementations, this should be done as quickly aspossible in order to improve the PRACH response time. FIG. 15 shows DLHARQ timing with UL/DL alignment at the controller. FIG. 16 shows ULHARQ timing with UL/DL alignment at the controller. Based on FIG. 16,the CU can send the PRACH response in subframe N+3.

In some implementations, the TA in the UE may be kept low, for exampleas low as the round-trip airlink delay, and one may accept the resultingreduced processing time in the controller. This may allow the radionetwork to use the Format 0 PRACH preamble, which uses less airlinkresources for PRACH, or not have any restrictions in scheduling due topotential collisions with PRACH, as described earlier.

Frame Alignment for TD-LTE

In frame advance in FDD-LTE. In TD-LTE, the frame structure is designedsuch that uplink and downlink transmissions do not overlap at the RU andUE receive antennas. A special subframe can be used to transition fromDL to UL transmission, as shown in FIG. 19. The special subframe startswith a few OFDM symbols of DL transmission, followed a silence gapinterval GP that lasts a few OFDM symbols and ends with 1 or 2 OFDMsymbols of UL transmission. The UL transmission in the special subframecan only carry SRS or PRACH (which needs two OFDM symbols). LTE standardsupports 9 different configurations for the special subframe as shown inthe Table 2.

TABLE 2 Subframe Configurations Supported by LTE. Special SubframeConfiguration DL P L Total 0 3 0 14 1 9 14 2 11 14 3 11 14 4 12 14 5 314 6 9 14 7 10 14 8 11 14

As in FDD, the UE advances the UL frame timing relative to the receivedDL timing by TA seconds. This aligns transmissions by different UEs atthe RU antennas. In TD-LTE, the maximum expected value of TA determinesthe gap interval GP. In order to avoid simultaneous DL and ULtransmissions at the UE or RU receive antennas, GP is selected such thatGP≥TA≥t_(RT), where t_(RT) represents the round-trip airlink propagationdelay between the UE and RU antennas.

As shown in FIG. 19, if GP<TA, the UE's UL transmission at the end ofthe special subframe will interfere with the reception of the DLtransmission in the beginning of the same special subframe. IfGP<TA−t_(RT), then the RUs DL transmission in the beginning of thespecial subframe will cause interference into the RUs reception of theUL transmission at the end of the special subframe. If TA<t_(RT), thenthe RUs DL transmission immediately following an UL-to-DL transitionwill interfere with the RUs reception of the UE's last UL subframetransmission before the UL-to-DL transition.

In some implementations, it is possible for the controller to choose TAto align DL and UL transmissions at the controller as in FDD in order topreserve the 3 ms processing time. The special subframe configurations 0or 5 can be used, which support a GP value (9 or 10 OFDM symbols) thatis large enough to avoid the UL-DL interference described above.Sometimes, the large value of GP can cause inefficiency on DLtransmissions.

In some implementations, a shorter TA value may be used for TD-LTE. InTD-LTE, the HARQ timing is different from that in FDD and depends on thespecific TDD frame configuration. Table 3 shows the minimum HARQ timingrequirements for the 9 different frame configurations that are supportedin the standard. The frame configuration is sent by the controller in aSIB message.

TABLE 3 Minimum HARQ Timing Requirements for 9 Different FrameConfigurations Subframe # 0 1 2 3 4 5 6 7 8 9 0 D S U U U D S U U UACK/NAK 4 6 4 7 6 4 6 4 7 6 Re-Transmission 6 4 6 4 4 6 4 6 4 4 TotalTime 10  10  10  11  10  10  10   10 11  10  1 D S U U D D S U U DACK/NAK 7 6 4 6 4 7 6 4 6 4 Re-Transmission 4 4 6 4 6 4 4 6 4 6 TotalTime 11  10  10  10  10  11  10  10  10  10  2 D S U D D D S U D DACK/NAK 7 6 6 4 8 7 6 6 4 8 Re-Transmission 4 4 4 4 4 4 4 4 4 4 TotalTime 11  10  10  8 12  11  10  10  8 12  3 D S U U U D D D D D ACK/NAK 411  6 6 6 7 6 6 5 5 Re-Transmission 4 4 4 4 4 4 4 4 4 4 Total Time 8 15 10  10  10  11  10  10  9 9 4 D S U U D D D D D D ACK/NAK 12  11  6 6 87 7 6 5 4 Re-Transmission 4 4 4 4 4 4 4 4 4 4 Total Time 16  15  10  10 12  11  11  10  9 8 5 D S U D D D D D D D ACK/NAK 12  11  6 9 8 7 6 5 413  Re-Transmission 4 4 4 4 4 4 4 4 4 4 Total Time 16  15  10  13  12 11  10  9 8 17  6 D S U U U D S U U D ACK/NAK 7 7 4 6 6 7 7 4 7 5Re-Transmission 8 7 6 4 4 7 6 6 7 5 Total Time 15  14  10  10  10  14 13  10  14  10 

For each frame configuration, Table 3 shows example DL (D), UL (U) andSpecial (S) subframes in a radio frame. Configurations 3-5 support asingle DL-UL transition and the other configurations support two DL-ULtransitions within a 10 ms radio frame. For each frame configuration,Table 3 also shows the number of subframes between the transmission ofthe shared channel data and the transmission of ACK/NAK by the receivingnode. In DL HARQ, the ACK/NAK time varies between 4 and 13 subframes.Sometimes the UE will have 3−TA ms processing time available, same as inFDD. In UL HARQ the ACK/NAK time varies between 4 and 7 subframes. WhenDL capacity requirements are higher than that on the UL, configurations2-5 can be used for in-building systems. In these configurations, theACK/NAK time is fixed at 6 subframes, 2 subframes longer than in FDD.This gives the controller 5−T_(RL)+t_(RT) seconds of processing time. IfTA is minimized by setting it equal to the round-trip airlink delay,e.g., TA=t_(RT), then the available processing time is 5−T_(RT). If TAis chosen to also compensate for the controller-RU round-trip delayT_(RT), e.g., TA=T_(RT)+t_(RT), then the available time is 5 subframes,which is 2 subframes longer than in FDD.

Table 3 also shows example retransmission times. It can be seen that theDL retransmission time varies between 4 and 8 subframes, but forconfigurations 3-5 it is always equal to 4, the same as in FDD. Theavailable processing time in the controller increases from 3-T_(R) to 3ms as TA is increased from t_(RT) to t_(RT)+T_(RT). This is the sametrade-off as in FDD. In the UL the retransmission time varies between 4and 7 subframes. In the worst-case of 4 subframes, the availableprocessing time in the UE is the same as in FDD.

In TD-LTE PRACH opportunities are allowed in UL subframes. PRACHopportunities may also be created in special subframes when at least 2OFDM symbols are assigned to PRACH (special subframe configurations5-8). But in this case, the available silence interval is 288 samples(at 20 MHz), or 9.375 ns, which limits the round-trip airlinkpropagation delay to 9.375 ns, or about 1.4 km. This shows that inin-building networks, special subframes can be used for PRACH when UL/DLframes are aligned at the RUs and reduced processing time that may beavailable in the controller in certain configurations is accepted. Theuse of PRACH in normal UL subframes is the same as in FDD, except inTD-LTE multiple PRACH opportunities can be supported in a singlesubframe.

Implementations

Although various assumptions are made for the purpose of explanation,the example implementations of the systems and methods described in thisdisclosure are not limited by these assumptions. Instead, theexplanation based on these assumptions can be readily generalized toother situations. For example, the numbers of RUs in each cell, thenumbers of antennas for each RU, and the numbers of cells in a networkcan vary, e.g., based on the network demands.

In an aspect, this disclosure features a communication system comprisingremote units and a controller. Each of the remote units may comprise oneor more radio frequency (RF) units to exchange RF signals with mobiledevices. At least some of the RF signals comprise information destinedfor, or originating from, a mobile device. The controller comprises oneor more modems and is connected to an external network. At least one ofthe modems is a baseband modem and is configured to pass first datacorresponding to the information. The at least one of the modems isconfigured to perform real-time scheduling of the first datacorresponding to the information. The controller is separated from theremote units by an intermediate network. The intermediate networkcomprises a switched Ethernet network over which second datacorresponding to the information is carried in frames between thecontroller and the remote units.

In another aspect, this disclosure features a communication systemcomprising remote units, a reference timing source, a controller, acontroller clock, and a remote unit clock. The remote units exchangeradio frequency (RF) signals with mobile devices. At least some of theRF signals comprise information destined for, or originating from, amobile device. The reference timing source is synchronized with acoordinated universal time (UTC) or a Global Positioning System (GPS).The controller comprises one or more modems and is connected to anexternal network. At least one of the modems is a baseband modem and isconfigured to pass first data corresponding to the information. Thecontroller is separated from the remote units by an intermediate networkover which second data corresponding to the information is transmittedin frames between the controller and the remote units. The second datacomprises baseband data. The controller clock is synchronized with thereference timing source. The controller clock provides timinginformation to the controller. The remote unit clock is synchronizedwith the controller clock. The remote unit clock provides timinginformation to a remote unit. The controller and the remote unit areconfigured to transmit time stamp messages to synchronize the controllerclock and the remote unit clock. The controller and the remote units areconfigured to transmit the time stamp messages by avoiding contentionbetween time stamp transmissions and baseband data transmissions orbetween time stamp transmissions of different remote units to thecontroller.

In another aspect, the disclosure features a communication systemcomprising remote units and a controller. The remote units exchangeradio frequency (RF) signals with mobile devices. At least some of theRF signals comprise information destined for, or originating from, amobile device. The controller comprises one or more modems and isconnected to an external network. At least one of the modems is abaseband modem and is configured to pass first data corresponding to theinformation. The controller is separated from the remote units by anintermediate network over which second data corresponding to theinformation is carried in frames between the controller and the remoteunits. The second data comprises baseband data and the intermediatenetwork is configured to transport in frames baseband data. At leastsome of the baseband data is compressed in a frequency domain. Theremote units and the controller are configured to compress the basebanddata for transmission over the intermediate network.

The aspects of the disclosure may also include one or more of thefollowing features. The intermediate network comprises multipleswitches. The external network comprises the Internet. The mobiledevices are cellular communication devices that communicate using thelong term evolution (LTE) standard. The remote units are configured toperform some modem functionality. The controller is devoid of RF radiofunctionality. The switched Ethernet network comprises multipleswitches. At least one of the multiple switches is connected to at leastone remote unit over a 1 gigabit/second Ethernet link. Each remote unitcomprises multiple RF antennas and is configured to transmit and/orreceive RF signals from one or more mobile devices simultaneously overone or more radio channels. The controller comprises one or moreprocessing devices, the one or more processing devices being programmedto associate one or more of the modems with one or more of the remoteunits to thereby configure communication cells that comprise one or moreremote units. The one or more processing devices are programmed toassociate one or more of the modems with one or more of the remote unitsto thereby configure the communication cells dynamically. The one ormore modems control a set of the remote units through the switchedEthernet network to form a cell, each remote unit in the cell comprisingone or more antennas, the one or more antennas being associated with acommon cell identifier. The common cell identifier comprises the longterm evolution (LTE) Cell-ID. All remote units associated with the cellmay be configured to communicate over a single long term evolution (LTE)channel. Each remote unit associated with the cell comprises a pair ofantennas, and at least two pairs of antennas of remote units associatedwith the cell are controllable to communicate with a single pair ofantennas on a single mobile device. Each remote unit associated with thecell comprises one or more antennas. Each antenna corresponds to avirtual antenna port. All antennas assigned to a same virtual antennaport simulcast a common signal. The remote units assigned to the samevirtual antenna port carry the same LTE downlink reference signalsassociated with the same virtual antenna port. The virtual antenna portincludes a Channel State Information Reference Signal (CSI-RS)scrambling ID. The mobile device sends more than one Channel StateInformation (CSI) feedback. Each of the antennas of the remote units isassigned to a different virtual antenna port. The remote units in thecell are synchronized to communicate using a same frequency. The remoteunits in the cell are configured to implement a network-basedsynchronization protocol to effect synchronization. The controllercomprises one or more processing devices, the one or more processingdevices being programmed to modify an association of one or more of themodems with one or more of the remote units to thereby re-configureexisting communication cells defined by one or more remote units.Re-configuring existing communication cells comprises splitting at leastone existing communication cell into two or more new communicationcells. Re-configuring existing communication cells comprises combiningat least two existing communication cells into a single newcommunication cell. The controller is configured to modify theassociation based on commands received from a management system. Thecontroller is configured to modify the association based on time-of-day.The controller is configured to modify the association based on changesin a distribution of demand for communication capacity. The cell isconfigured to virtually split to send data to two or more mobile deviceson the same resources without substantial interference based on radiofrequency isolation between the two or more mobile devices. Theresources are time-frequency resources of long term evolution (LTE). Thecontroller is configured to determine which mobile devices to send dataon the same resource based on signals received from the mobile devices.The mobile devices comprise receivers and the data sent to the receiversby the remote units in the cell is not on the time-frequency resource.The cell is configured to virtually split to receive information fromtwo or more mobile devices on the same resources without substantialinterference based on radio frequency isolation between the two or moremobile devices. Two or more mobile devices use the same demodulationreference sequence. The two or more mobile devices use the same PUCCHresource comprised of a cyclic shift and orthogonal cover code. Thecontroller is configured to detect RACH preamble transmissions from thetwo or more mobile devices sent in the same PRACH opportunity. Thecontroller comprises one or more processing devices, the one or moreprocessing devices being programmed to associate one or more additionalmodems with one or more of the remote units in response to a change indemand for communication capacity. In response to a decrease in demandfor network capacity, the one or more processing devices are programmedto consolidate the one or more remote units among a decreased number ofthe one or more modems. The cell is a first cell and the modem is afirst modem; and the one or more modems comprise a second modemprogrammed to control a second set of the remote units through theswitched Ethernet network to form a second cell, each RF unit in thesecond cell comprising one or more second antennas, the one or moresecond antennas being associated with a second common cell identifier.The first cell and the second cell comprise different numbers of remoteunits, different shapes, and/or transmit radio signals coveringdifferent sized areas. The controller comprises one or more processingdevices, the one or more processing devices being programmed toassociate the first and second modems with different remote units inorder to dynamically change shape and/or an area covered by each of thefirst cell or the second cell. The first and second modems areco-located with the controller, and the controller coordinates thetransmissions of the first and second modems to reduce interferencebetween the first and second cells. At least one remote unit isconfigured to exchange Wi-Fi signals with a corresponding device. Thecontroller comprises one or more processing devices, the one or moreprocessing devices being programmed to receive second data from theswitched Ethernet network and to process the second data to generatefirst data. At least some of the remote units are configured to receivepower through the switched Ethernet network. The controller and theremote units are configured to communicate using the IEEE1588 protocol.The communication system also includes a network manager incommunication with the controller that directs operation of thecontroller. The external network comprises an operator's core networkand the network manager is located in the operator's core network. Thenetwork manager is located locally with respect to the controller. Twoor more remote units are configured to send the second data to a mobiledevice on two or more RF channels so that the mobile receives the seconddata simultaneously from the two or more remote units. The controller isconfigured to aggregate communication from different channels betweenthe controller and the remote units and the controller and the externalnetwork to process the first data and to send the second data to theremote units.

The aspects of the disclosure may also include one or more of thefollowing features. The first data comprises Internet Protocol (IP) dataand the controller is configured to perform real-time media accesscontrol of the IP data corresponding to the information. The referencetiming source comprises a GPS receiver. The GPS receiver is located inthe controller. The controller and the remote units are configured toexchange time stamps using the IEEE 1588 protocol. The controller andthe remote units comprise a system-on-chip to generate and process thetime stamp messages. The intermediate network is a switched Ethernetnetwork. The remote unit uses the time stamp messages to estimate andcorrect an error of the remote unit clock. The estimation is based on apriori knowledge about downlink and uplink time stamp delays. The apriori knowledge about the downlink and uplink time stamp delayscomprises a ratio of the downlink time stamp delay to the uplink timestamp delay. The a priori knowledge about the downlink and uplink timestamp delays comprises a ratio of an average downlink time stamp delayto an average uplink time stamp delay. The error comprises a timingphase error and the remote unit is configured to estimate the timingphase error by weighting and/or offsetting measured time stamps in theuplink and the downlink according to the a priori knowledge. The timestamp messages are transmitted with high priority according to the IEEE802.1q protocol. The time stamp messages and the baseband data aretransmitted on different virtual local area networks (VLANs). The timestamp messages and the baseband data are transmitted on the same virtuallocal area network (VLAN) using different priority markings of the IEEE802.1q protocol. The baseband data and the time stamp messages aretransmitted using dedicated Ethernet ports and dedicated Ethernet linksof the switched Ethernet network. The communication system comprises aplurality of controllers and one of the controllers is a mastercontroller and is configured to transmit the time stamp messages withremote units associated with the master controller and with remote unitsassociated with the other controllers of the plurality of controllers.The controller is configured to advance in time a subframe of basebanddata to be delivered to a remote unit to compensate a time delay betweenthe remote unit clock and the controller clock. The controller isconfigured to advance in time the subframe of baseband data for apre-determined amount. The pre-determined amount is determined based ona time delay for transmitting the baseband data over the intermediatenetwork. The controller is configured to send information to the mobiledevices for the mobile devices to advance a timing phase of the RFsignals to be transmitted to the remote units relative to the RF signalsreceived by the mobile devices from the remote units. The controller isconfigured to increase processing time available to the controller forthe controller to process the baseband data transmissions by choosing anamount of the timing phase to be advanced to be greater than a timedelay for transmitting RF signals in a round trip between a remote unitand a mobile device. A remote unit is configured to advance in timesubframes of the baseband data to be transmitted to the controller. Theremote units are configured to communicate with the controller on acommunication channel, and a frequency of the communication channel isderived from the controller clock. The controller clock comprises acrystal oscillator configured to generate clocks for baseband processingin the controller. The remote unit clock comprises a crystal oscillatorconfigured to generate clocks for analog-digital-analog converters(A/D/As), RF synthesizers, and/or baseband processing in each remoteunit. The controller and the remote unit are configured to transmit timestamp messages in multiple round-trips between the controller and theremote unit. The remote unit is configured to adjust the remote unitclock based on one of the transmissions in multiple round-trips that isdeemed to be most reliable to correct an offset between the controllerclock and the remote unit clock. The one of the transmissions inmultiple round-trips that is deemed to be most reliable comprises atransmission that predicts a smallest offset between the controllerclock and the remote unit clock. The remote unit is configured to not tomake any correction to the remote unit clock when an estimate of anoffset between the controller clock and the remote unit clock based onthe transmissions of the time stamp messages is deemed to be unreliable.The estimate of the offset is deemed to be unreliable when the estimateexceeds a pre-configured threshold. The controller clock is in directcoupling with the reference timing source and the remote unit clock isnot in direct coupling with the reference timing source.

The aspects of the disclosure may also include one or more of thefollowing features. A rate of transmission of the baseband data over theintermediate network is at most 1 Gb/s. The baseband data is representedby complex-valued signals having real and imaginary components, and thecontroller is configured to compress the baseband data by quantizing thecomplex-valued signals in the frequency domain to produce quantizedbaseband data, and to transmit binary data representative of thequantized baseband data to the remote units. The remote units areconfigured to reconstruct the quantized baseband data upon receipt ofthe compressed baseband data. The remote units are configured to applyan inverse fast Fourier transform on the reconstructed baseband data.The controller is configured to quantize the baseband data in thefrequency domain using a quantizer having a fixed rate and a fixed stepsize. The controller is configured to quantize independently the realand imaginary components of the baseband data in the frequency domain.The controller is configured to send information about the fixed rateand the fixed step size to the remote units when the remote units andthe controller are connected. The controller is configured to quantizethe baseband data in the frequency domain using a quantizer having afixed rate and an adjustable step size. The controller is configured tosend side information about the fixed rate and a step size to a remoteunit once per subframe. The controller is configured to quantize thebaseband data in the frequency domain using a quantizer having a rateand a step size. The rate and the step size both are adjustable. Thecontroller adjusts the step size according to energy of the quantizedbaseband data. The controller adjusts the rate according to a modulationand coding scheme of the baseband data. The RF signals are compatiblewith the long term evolution (LTE) standard. The controller isconfigured to send side information about the rate of the quantizer to aremote unit for each of plural resource element groups (REG) andphysical resource blocks (PRB) in each orthogonal frequency-divisionmultiplexing (OFDM) symbol of a subframe. The controller is configuredto compress the baseband data by not sending to the remote units anydata for unused resource element groups (REGs) or physical resourceblocks (PRBs) in each orthogonal frequency-division multiplexing (OFDM)symbol of the baseband data. The baseband data in the frequency domainbelongs to, or is derived from, a discrete-amplitude signalconstellation, and the controller is configured to compress the basebanddata without quantization by sending binary data representing thediscrete-amplitude signals to the remote units. The discrete-amplitudesignal constellation comprises a quadrature amplitude modulation (QAM)signal constellation. The RF signals carry orthogonal frequency-divisionmultiplexing (OFDM) symbols, and the controller is configured to sendthe binary data to the remote units in the same order as thecorresponding OFDM symbols are to be transmitted by the remote unitsover the air to the mobile devices. The remote units are configured tocompress the baseband data by quantizing the baseband data in thefrequency domain to produce quantized baseband data, and to transmitbinary data representative of the quantized baseband data to thecontroller. A remote unit is configured to receive data in time domainfrom the mobile device and to apply a fast Fourier transform to the datain the time domain to produce the baseband data in the frequency domain.A remote unit is configured to quantize the baseband data in thefrequency domain using a quantizer having a fixed rate and a fixed stepsize. A remote unit is configured to quantize the baseband data in thefrequency domain using a quantizer having a fixed rate and an adjustablestep size. The frames of the baseband data comprise orthogonalfrequency-division multiplexing (OFDM) symbols and the remote unit isconfigured to select a step size based on an average energy of thequantized baseband data. The average energy is an average of energies ofbaseband data that belong to a long term evolution (LTE) channel. Theremote unit is configured to select a step size based on a distributionof the baseband data in the frequency domain. The remote unit isconfigured to send side information about the quantizer to thecontroller for the controller to reconstruct the received quantizedbaseband data. A remote unit is configured to quantize the baseband datain the frequency domain using a quantizer having a rate and a step size,the rate and the step size both being adjustable. The frames of thebaseband data comprise subframes comprising LTE physical resource blocks(PRBs), and the remote unit is configured to adjust the rate of thequantizer on a per PRB basis. The remote unit is configured to select aquantizer rate based on a modulation and coding scheme of the basebanddata determined by the controller. The remote units are configured toquantize the baseband data using quantizers having adjustable rates. Thequantizer rates for the baseband data are adjusted according to the LTEresource blocks. The quantizer rates are chosen to be zero to purgetransmissions of the baseband data for some of the resource blocks. Thecontroller is configured to send side information to the remote unitsand the information is used by the remote units to determine thequantizer rates. The controller is configured to determine the sideinformation to be sent to the remote units based on information receivedfrom the mobile devices. The controller is configured to determine theside information based on a target signal-to-noise plus interferenceratio (SINR) at the controller. The information received from the mobiledevices corresponds to LTE Sounding Reference Signal (SRS) transmissionsby the mobile devices. The information received from the mobile devicescorresponds to LTE Physical Random Access Channel (PRACH) transmissionsby the mobile devices. The information received from the mobile devicescorresponds to uplink transmission on the Physical Uplink Shared Channel(PUSCH) by the mobile devices. A remote unit comprises two or morereceiver antennas for receiving the RF signals from the mobile devices,and the remote unit is configured to quantize the baseband datacorresponding to the different antennas using different quantizers. Thequantizers for different antennas have different step sizes. Thequantizers for different antennas have different step sizes anddifferent rates. The different rates are determined by the controller.The controller is configured to send side information to the remote unitto indicate the determined quantizer rate for each receive antenna. Aremote unit comprises two or more receiver antennas for receiving the RFsignals from the mobile devices. The remote unit is configured toquantize the baseband data using a quantizer having a rate selectedbased on correlation of the RF signals received at different receiversof the remote unit. The controller is configured to determine acoefficient based on the correlation of the RF signals and to determinethe rate of the quantizer using the coefficient. The remote unit isconfigured to determine the rate of the quantizer using a coefficientdetermined by the controller based on the correlation of the RF signals.The remote unit is configured to determine a coefficient based on thecorrelation of the RF signals and to determine the rate of the quantizerusing the coefficient. All baseband data except for those correspondingto Physical Random Access Channel (PRACH) transmissions from a mobiledevice is compressed in the frequency domain. A remote unit isconfigured to compress the baseband data by quantizing the receivedPRACH transmissions after performing a correlation in the frequencydomain. The remote unit is configured to compress the baseband data byquantizing the received PRACH transmissions in a time-domain afterconverting an output of the correlation back into the time domain. Atleast one modem of the controller is configured to execute real-timemedia access control (MAC) functions for the IP data corresponding tothe information.

Any two or more of the features described in this patent application maybe combined to form implementations not specifically described in thispatent application.

All or part of the foregoing may be implemented as a computer programproduct comprised of instructions that are stored on one or morenon-transitory machine-readable storage media, and that are executableon one or more processing devices. All or part of the foregoing may beimplemented as an apparatus, method, or system that may include one ormore processing devices and memory to store executable instructions toimplement functionality.

All or part of the processes described herein and their variousmodifications (hereinafter referred to as “the processes”) can beimplemented, at least in part, via a computer program product, e.g., acomputer program tangibly embodied in one or more information carriers,e.g., in one or more tangible, non-transitory machine-readable storagemedia, for execution by, or to control the operation of, data processingapparatus, e.g., a programmable processor, a computer, or multiplecomputers

A computer program can be written in any form of programming language,including compiled or interpreted languages, and it can be deployed inany form, including as a stand-alone program or as a module, component,subroutine, or other unit suitable for use in a computing environment. Acomputer program can be deployed to be executed on one computer or onmultiple computers at one site or distributed across multiple sites andinterconnected by a network.

Actions associated with implementing the processes can be performed byone or more programmable processors executing one or more computerprograms to perform the functions of the calibration process. All orpart of the processes can be implemented as, special purpose logiccircuitry, e.g., an FPGA (field programmable gate array) and/or an ASIC(application-specific integrated circuit).

Processors suitable for the execution of a computer program include, byway of example, both general and special purpose microprocessors, andany one or more processors of any kind of digital computer. Generally, aprocessor will receive instructions and data from a read-only storagearea or a random access storage area or both. Elements of a computer(including a server) include one or more processors for executinginstructions and one or more storage area devices for storinginstructions and data. Generally, a computer will also include, or beoperatively coupled to receive data from, or transfer data to, or both,one or more machine-readable storage media, such as mass storage devicesfor storing data, e.g., magnetic, magneto-optical disks, or opticaldisks. Machine-readable storage media suitable for embodying computerprogram instructions and data include all forms of non-volatile storagearea, including by way of example, semiconductor storage area devices,e.g., EPROM, EEPROM, and flash storage area devices; magnetic disks,e.g., internal hard disks or removable disks; magneto-optical disks; andCD-ROM and DVD-ROM disks.

Elements of different implementations described herein may be combinedto form other implementations not specifically set forth above. Elementsmay be left out of the processes, computer programs, Web pages, etc.described herein without adversely affecting their operation.Furthermore, various separate elements may be combined into one or moreindividual elements to perform the functions described herein.

Any of the features described herein may be combined, as appropriate,with any of the features described in U.S. Provisional Application No.62/009,653, which is incorporated herein by reference. Any of thefeatures described herein may be combined, as appropriate, with any ofthe features described in U.S. Provisional Application No. 62/051,212,which is incorporated herein by reference.

What is claimed is:
 1. A communication system comprising: remote unitsto exchange radio frequency (RF) signals with mobile devices, at leastone of the RF signals comprising information destined for, ororiginating from, a mobile device of the mobile devices, and wherein theremote units comprise radio frequency transceivers to communicate withthe mobile devices; and a controller comprising a real-time schedulerfor assigning the mobile devices to time-frequency resources; whereinthe controller is configured to determine remote unit transmission orreception needs of the mobile devices by estimating signal levels fromthe mobile devices and representing the remote unit transmission orreception needs of the mobile devices with numerical values; wherein thereal-time scheduler assigns the time-frequency resources to the mobiledevices based on the numerical values, assigning a same time-frequencyresource to two or more mobile devices based on RF isolation between thetwo or more mobile devices, the RF isolation determined using numericalvalues for a respective signal received from each of the two or moremobile devices at each of the remote units.
 2. The communication systemof claim 1, wherein the remote unit transmission or reception needs ofthe mobile device are determined based on at least one of: estimates ofsignal loss between each of the remote units and the mobile device; andtraffic load seen on each of the remote units.
 3. The communicationsystem of claim 2, wherein the signal loss is estimated based on atleast one uplink transmission from the mobile device to the remoteunits.
 4. The communication system of claim 3, wherein at least oneuplink transmissions correspond to at least one of: Sounding ReferenceSignal (SRS) transmissions; Physical Random Access Channel (PRACH)transmissions; Physical Uplink Control Channel (PUCCH) transmissions;and Physical Uplink Shared Channel (PUSCH) transmissions.
 5. Thecommunication system of claim 1, wherein the numerical values arederived from uplink measurements.
 6. The communication system of claim5, wherein the numerical values are binary, each taking a value of 0or
 1. 7. The communication system of claim 6, wherein the numericalvalues take on values from a finite number of levels greater than
 2. 8.The communication system of claim 7, wherein for each mobile device thenumerical values are used to form a quantized signature vector and twomobile devices are allowed to be scheduled on the same frequencyresource when a sum of their quantized signature vectors have nocomponent that exceeds a preset threshold.
 9. The communication systemof claim 6, wherein the respective numerical values for each mobiledevice are used to form a respective quantized signature vector, eachquantized signature vector having a respective entry for a respectiveestimated signal level at a respective one of the remote units, andwherein the controller is configured to determine, based on thequantized signature vectors, that the two or more mobile devices can bescheduled on the same time-frequency resource for communication bydetermining that the quantized signature vectors for the two or moremobile devices are orthogonal.
 10. The communication system of claim 9,wherein the respective quantized signature vector for the mobile deviceis determined using a threshold signal-to-interference-plus noise ratio(SINK).
 11. The communication system of claim 1, wherein the numericalvalues are determined based, at least in part, on locations of themobile devices within a communication cell, wherein the remote unitseach belong to at least the communication cell.
 12. The communicationsystem of claim 1, wherein assigning the same time-frequency resource tothe two or more mobile devices results in at least one of: differentremote units in a communication cell transmitting to different mobiledevices on the same time-frequency resource; a first set of remote unitsnot transmitting to any of the mobile devices; and a second set ofremote units transmitting simultaneously to multiple mobile devices. 13.The communication system of claim 12, wherein the remote unitstransmitting simultaneously to the multiple mobile devices have areduced transmit power.
 14. The communication system of claim 1, whereinthe controller further reduces transmission power to certain mobiledevices that it determines to be near a remote unit thereby increasingRF isolation between the mobile devices.
 15. The communication system ofclaim 14, wherein the controller makes the determination based onmeasurements of uplink transmissions of the mobile devices at the remoteunits.
 16. The communication system of claim 15, wherein the uplinktransmissions are SRS, PUCCH, PRACH or PUSCH transmissions.
 17. Thecommunication system of claim 1, wherein the controller is configured todetermine bit rates at which data is to be transmitted to and from thetwo or more mobile devices using the assigned time-frequency resources.18. The communication system of claim 17, wherein determining a bit ratefor communication between the mobile device and one of the remote unitscomprises: receiving, from the remote units, measurements on an uplinkcontrol channel, and using such measurements in determining the bitrate.
 19. The communication system of claim 18, wherein determining thebit rate includes uncertainty due to small-scale fading.
 20. Thecommunication system of claim 19, wherein determining the bit rate for acommunication from a remote unit to the mobile device comprises:receiving, from the mobile device, feedback on a success or failure ofpast data transmissions, and using such information in determining thebit rate.
 21. The communication system of claim 20, wherein the feedbackis Hybrid ARQ (HARQ) feedback.
 22. The communication system of claim 21,wherein the HARQ feedback is ignored when the mobile device's dominantinterferer has changed.
 23. The communication system of claim 17,wherein determining a bit rate for a communication from a remote unit tothe mobile device comprises one of: receiving, from at least some of themobile devices, multiple channel state information (CSI) feedback andusing the CSI feedback in determining the bit rate; and receiving, fromthe at least some of the mobile devices, multiple channel state feedbackincluding interference measurement and using channel state feedback indetermining the bit rate.
 24. The communication system of claim 23,wherein the interference measurement is based on a Channel StateInformation Reference Signal (CSI-RS).
 25. The communication system ofclaim 24, wherein the mobile device reports multiple interferencemeasurements for different interference scenarios.
 26. The communicationsystem of claim 1, wherein the controller is configured to manage uplinkcontrol channel processing load on the remote units, comprisingadjusting periods for transmissions from some mobile devices based on acommunication traffic load in a communication cell, wherein the remoteunits belong to at least the communication cell.
 27. The communicationsystem of claim 26, wherein the communication traffic load is measuredbased on a number of connected users.
 28. The communication system ofclaim 1, wherein the same time-frequency resource is assigned for atleast one of: downlink transmissions from different remote units to thetwo or more mobile devices; a first set of uplink transmissions from thetwo or more mobile devices to the remote units without substantialinterference; and a second set of uplink transmissions from the two ormore mobile devices to one or more remote units whose received signalsare jointly processed for reliable detection.